ARM: bcm_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 bcm_defconfig to work with new menu levels.
- multi_v7_defconfig in another patch for merge purposes as per
Olof Johansson's request

Signed-off-by: Scott Branden <sbranden@broadcom.com>