[MIPS] lib_mips/board.c: Add nand_init

This patch adds the standard 'nand_init()' call to the mips generic
'board_init_r()' call, bringing MIPS in line with the other architectures.

Signed-off-by: Jason McMullan <mcmullan@netapp.com>
Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>