[ARM] i.MX6 clocking code calls to Xvisor API corrected.

The 'vmm_sprintf' function does not return an error code, but the number of
buffer written bytes.
The 'vmm_printf', called through 'pr_err', does not support the '%ld', as 'l'
is not a length modifier, but a conversion specifier.
[DRIVER] i.MX6 serial warning corrected.

A warning on a bitwise OR operation has been corrected.
[ARM] i.MX6 CCM device added in the device tree, required for LP clocking.

As the CCM was not declared in the device tree, the associated driver, in
arch/arm/cpu/board/common/imx/clk-imx6q.c was not initialized. Thus, the Low
Power Manager clocking did not set the low power wait mode as clocked, state in
which a WFI set the processor to. In consequence, when the first proc_do_idle
was called, the JTAG probe lost its control over the processor.
The scheduling was functioning though, as the EPIT timer was running.
[DRIVERS] i.MX6Q UART TX interrupt corrected and menuconfig option added.

If the menuconfig option is set, the TX interrupt is enabled, allowing to
sleep when the TX FIFO is full, and to be awaken when it has been cleared.
[CORE] Avoid the "always_inline function might not be inlinable" warning.

GCC displays a warning about the fact a function may not be inlined even if
told so, if no optimization is asked for. To avoid this, as explained in the
GCC manual :
Generally, functions are not inlined unless optimization is specified. For
functions declared inline, this attribute inlines the function even if no
optimization level is specified.

Adding "inline" correct this warning.