ARM: bcm: clean up config and build targets

ARM: bcm: clean up config and build targets

Currently CONFIG_ARCH_BCM_MOBILE is used to select all (both)
Broadcom mobile SoC families.  Instead, use that only as a config
menu switch, and define specific symbols like ARCH_BCM_281XX to
select a particular SoC family.  If ARCH_BCM_MOBILE is selected, all
of the SoCs will be selected by default, but this way each can be
disabled individually as well.

Note that BCM281xx and BCM21664 both require the SMC and L2 cache
control code, so that code will be built based on ARCH_BCM_MOBILE.

Signed-off-by: Alex Elder <elder@linaro.org>
[mporter: added ARM: to the description]
Signed-off-by: Matt Porter <mporter@linaro.org>