mx5: lowlevel_init.S: Split init_clock macro

init_clock is currently shared between mx51 and mx53 and it contains lots of
ifdef's which makes it really hard to follow the code.

Split the init_clock between mx51 and mx53 to allow easier readability.

No functional changes are made.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>