rt2800: add rt2800_init_bbp_305x_soc subroutine

New routine for SOC specific BBP initialization, empty for now.

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>