clk: bcm2835: Add a driver for the auxiliary peripheral clock gates.

There are a pair of SPI masters and a mini UART that were last minute
additions.  As a result, they didn't get integrated in the same way as
the other gates off of the VPU clock in CPRMAN.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>