clk: bcm2835: expose raw clock-registers via debugfs

For debugging purposes under some circumstance
it helps to be able to see the actual clock registers.

E.g: when looking at the clock divider it is helpful to
see what the actual clock divider is.

This patch exposes all the clock registers specific to each
clock/pll/pll-divider via debugfs.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 96bf9c69)