clk: bcm: Make BCM2835 clock drivers selectable

Make the BCM2835 clock driver selectable by other
architectures/platforms. ARCH_BRCMSTB will be selecting that driver in
the next commit since new chips like 7211 use the same CPRMAN clock
controller that this driver supports.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>