ARM: bcm2835: add stub clock driver

This patch adds a minimal stub clock driver for the BCM2835. Its sole
purpose is to allow the PL011 AMBA clk_get() API calls to provide
something that looks enough like a clock that the driver probes and
operates correctly.

This patch was extracted from git://github.com/lp0/linux.git

 branch
rpi-split as of 2012/09/08, and modified as follows:

* Reworked to call clk_register_fixed_rate(), and clk_register_clkdev()
  rather than using static data to represent the clocks.
* Moved implementation to drivers/clk/.
* Modified .dev_id for UART clocks to match UART DT node names.
* s/bcm2708/bcm2835/.
* Modified device tree vendor prefix.

Signed-off-by: Chris Boot <bootc@bootc.net>
Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Dom Cobley <popcornmix@gmail.com>
Signed-off-by: Dom Cobley <dc4@broadcom.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Mike Turquette <mturquette@linaro.org>