[MIPS] RB532: Do not define registers that are already defined

Use the register definitions of the MPMC controller from
mach-rc32434/rb.h instead of redefining them.

Signed-off-by: Florian Fainelli <florian.fainelli@telecomint.eu>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>