[DRIVERS] i.MX SPI driver completion modified.

i.MX SPI IRQ are disabled too early, and led to an infinite wait on the
"wait_for_completion" in the "spi_imx_transfer" function.
A temporary hack is to wait with a timeout, reenabling the IRQ on a timeout.
[DRIVERS] Correcting i.MX SDHCI device tree property function calls.

The property length parameter must be set, as the NULL pointer is not checked.
[DRIVERS] i.MX serial driver TX FIFO value corrected.

The TX FIFO trigger level must be set with a correct value, even without using
TX interruptions.
[DRIVERS] Use the SDHCI board controller specific clock setting function.

The SDHCI driver clock setting function should only use the board controller
specific one if it is set, as the clock rate can be specific (as it is on the
i.MX6 board).
[DRIVERS] Spin locking SDHCI driver functions.
[ARM] Compiling with -O1 optimization level.
Improving section declaration macro name.
[ARM] Disabling some unused device tree node for debugging.
[CORE] Device tree device status retrieving function updated to match Linux's

Since Linux defines a function ("of_device_is_available") to retrieve status,
this patch update the Xvisor equivalent function to match Linux's name and
behavior.
[LIBS] Radix tree library missing the "vmm_compiler" header inclusion

Since the radix tree library is using "likely", an inclusion to vmm_compiler is
necessary.
[CORE] Correcting warnings on "vmm_devdrv_get_data" argument.

Some functions are passing a constant vmm_device structure argument to
"vmm_devdrv_get_data", leading to warnings. As this function does not modify
its argument, this patch defines the argument as constant.
[CORE] Add casting on the tested pointer in the "VMM_IS_ERR_VALUE" macro.

As we test pointers with "VMM_IS_ERR_VALUE", the argument should be cast to
an unsigned long before the test with "VMM_MAX_ERRNO".
[DRIVERS] Clock conf support added.
[DRIVERS] Add clock name retrieving method.
[DRIVERS] Add the PWM API and i.MX PWM controller support.

This patch add the support for PWM devices and the i.MX PWM controller driver
using it.
[ARM] i.MX6 PWM 4 device added in tree.

This patch adds the PWM 4, used by the LVDS backlight.
[DRIVERS] Backlight and PWM backlight API added.

This patch adds the backlight and the PWM driven backlight device support.
[COMMANDS] Backlight commands added.

This patches adds backlight a command to list backlight devices, and one to set
the backlight brightness.
[ARM] i.MX6 backlight device added.

This adds the backlight device used by Nitrogen6x (i.MX6).
This should be moved to a dedicated device tree file.
[COMMANDS] Improving backlight device commands by checking brightness bounds.