clk: bcm2835: probe for fixed-clock in device tree

This will allow fixed-clocks to be registered in device tree, which in
turn will allow clock clients to look up their clocks in device tree.
The first driver to use this feature will be the bcm2835 SDHCI/MMC
driver. In the long run, it is likely these fixed clocks will be
replaced with an actual clock implementation which speaks to the
VideoCore processor, but this change should be transparent to clock
consumers.

Cc: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>