pxa: Save lr register in relocate_code function

When u-boot is compiled for PXA25x processor, pxa/start.S is calling
cpu_init_crit by BL instruction. BL is overwriting lr register so
relocate_code is going into infinite loop. This patch preservs lr
register in r12 before calling cpu_init_crit and after function returns
restores it.

Signed-off-by: Lukasz Dalek <luk0104@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>