powerpc/mpc83xx: Zero boot_flags arg for calling board_init_f()

The argument boot_flags of board_init_f() is not used at all in the
powerpc specific board.c init sequence. Now with the generic init
sequence, this boot_flags arg is used by board_init_f().

This patch sets the r3 register that is used to pass the boot_flags
argument from the start.S board_init_f() call to 0 prior to the function
call to avoid unknown content to end up in gd->flags.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Reviewed-by: York Sun <yorksun@freescale.com>