spi: Allow selecting BCM2835 SPI controllers on ARCH_BRCMSTB

ARCH_BRCMSTB platforms have the BCM2835 SPI controllers (normal and
auxiliary), allow selecting the two drivers on such platforms.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Mark Brown <broonie@kernel.org>