[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.
[DRIVERS] TEMPORARY: Disable DMA usage for SDHCI drivers.

As the i.MX6 DMA channels are not supported yet, disable their usage for now
in the SDHCI driver.
[ARM] Compiling with -O1 optimization level.
Improving section declaration macro name.
[ARM] Disabling some unused device tree node for debugging.