clk: meson: mpll: correct N2 maximum value

Gxbb datasheet says N2 maximum value is 127 but the register field is
9 bits wide, the maximum value should 511.

Test shows value greater than 127, all the way to 511, works well

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20170309104154.28295-9-jbrunet@baylibre.com