ARM: multi_v7_defconfig: remove one level of menu from Kconfig

remove menu "Broadcom Mobile SoC Selection"
This requires:
- selecting ARCH_BCM_MOBILE based on SoC selections
- fixup multi_v7_defconfig to work with new menu levels of mach-bcm.

Signed-off-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>