mpc83xx: use r4 instead of r2 in lock_ram_in_cache and unlock_ram_in_cache

This is needed in unlock_ram_in_cache() because it is called from C and
will corrupt the small data area anchor that is kept in R2.

lock_ram_in_cache() is modified similarly as good coding practice, but
is not called from C.

Signed-off-by: Nick Spence <nick.spence@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>