On axg, the rate of the mpll is stuck as if sdm value was 4 and could not
change (expect for mpll2 strangely). Looking at the vendor kernel, it
turns out a new magic bit from the undocumented HHI_PLL_TOP_MISC register
is required.

Setting this bit solves the problem and the mpll rates are back to normal

Fixes: 78b4af31 ("clk: meson-axg: add clock controller drivers")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>