clk: meson: gxbb: mpll: use rw operation

Use read/write operations for the mpll clocks instead of the
read-only ones.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20170309104154.28295-7-jbrunet@baylibre.com