drm/pl111: Register the clock divider and use it.

This is required for the panel to work on bcm911360, where CLCDCLK is
the fixed 200Mhz AXI41 clock.  The rate set is still passed up to the
CLCDCLK, for platforms that have a settable rate on that one.

v2: Set SET_RATE_PARENT (caught by Linus Walleij), depend on
    COMMON_CLK.
v3: Mark the clk_ops static (caught by Stephen).

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170508193348.30236-1-eric@anholt.net


Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>