MIPS: bootm: Add fixup of '/memory' node.

MIPS arch do not update 'reg' property of /memory node.
As a result Linux bootup will not work unless board.dts
file contains right /memory offset-size information or
board implements required memory fixup.
Fixing by renaming (unused) _arch_fixup_memory_node_ to
_arch_fixup_fdt_ in arch/mips/lib/bootm.c inline with ARM arch.

Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>