ARM: bcm: Restrict ARCH_BCM selection to ARCH_MULTI_V6_V7

Currently, Broadcom SoC options are shown whenever ARCH_MULTIPLATFORM
is enabled. Restrict this so they are only seen when ARCH_MULTI_V6_V7
variants are enabled.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
[mporter: added commit log message]
Signed-off-by: Matt Porter <mporter@linaro.org>