MIPS: initialize board_init_f() argument to zero.

Argument boot_flags of board_init_f() should be set to 0 as
$a0 may be utilized in lowlevel_init() or mips_cache_reset()
or previous stage boot-loader.

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