kernel: add missing config option

brcm47xx does not use CONFIG_B53_SPI_DRIVER, but it could be selected if spi is build as a module

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>


git-svn-id: svn://svn.openwrt.org/openwrt@37656 3c298f89-4303-0410-b956-a3cf2f4a3e73