clk: bcm2835: Register the DSI0/DSI1 pixel clocks.

The DSI pixel clocks are muxed from clocks generated in the analog phy
by the DSI driver.  In order to set them as parents, we need to do the
same name lookup dance on them as we do for our root oscillator.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
(cherry picked from commit 8a39e9fa)