serial: mxs: spl: Remove empty serial_* functions from SPL code

Remove the empty bodies from serial_* functions from MXS SPL code.
These empty implementations are now in common/serial.c instead so
declaring them also in the SPL code would cause a colision once
serial multi is enabled unconditionally.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Stefano Babic <sbabic@denx.de>