mmc: rename ARCH_BCM to ARCH_BCM_MOBILE

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>
Signed-off-by: Chris Ball <chris@printf.net>