rename ARCH_BCM to ARCH_BCM_MOBILE (mach-bcm)

Currently ARCH_BCM has been used for Broadcom
Mobile V7 based SoCs. In order to allow other Broadcom
SoCs to also use mach-bcm directory and files, this patch
renames the original ARCH_BCM to ARCH_BCM_MOBILE, and
uses ARCH_BCM to define any Broadcom chip residing
in mach-bcm directory.

Signed-off-by: Christian Daudt <bcm@fixthebug.org>
Acked-by: Olof Johansson <olof@lixom.net>

Changes from v2:
 - switch ARCH_MULTIPLATFORM from select to depends
 - remove 'default y' from BCM_MOBILE

Changes from v1:
 - fix alpha ordering in dts/Makefile
 - break into 4 patches for separate subsys