clk: loongson2: Support scale clocks with an alternative mode

LS2K0300 and LS2K1500 ship scale clocks with an alternative mode.
There's one mode bit in clock configuration register indicating the
operation mode.

When mode bit is unset, the scale clock acts the same as previous
generation of scale clocks. When it's set, a different equation for
calculating result frequency, Fout = Fin / (scale + 1), is used.

This patch adds frequency calculation support for the scale clock
variant. A helper macro, CLK_SCALE_MODE, is added to simplify
definitions.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>