clk: meson: axg: add hifi pll clock

Add the hifi pll to the axg clock controller. This clock maybe used as an
input of the axg audio clock controller. It uses the same settings table
as the gp0 pll but has a frac parameter allowing more precision.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>