arm: mxs: Add serial console support into SPL

Add support for serial console into the i.MX23/i.MX28 SPL. A full,
uncrippled serial console support comes very helpful when debugging
various spectacular hardware bringup issues early in the process.
Because we do not use SPL framework, but have our own minimalistic
SPL, which is compatible with the i.MX23/i.MX28 BootROM, we do not
use preloader_console_init(), but instead use a similar function to
start the console. Nonetheless, to avoid blowing up the size of the
SPL binary, this support is enabled only if CONFIG_SPL_SERIAL_SUPPORT
is defined, which is disabled by default.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>