clk: ingenic: Support overriding PLLs M/N/OD calc algorithm

SoC-specific code can now provide a callback if they need to compute the
M/N/OD values in a specific way.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: 周琰杰 (Zhou Yanjie)<zhouyanjie@wanyeetech.com>   # on CU1000-neo/X1000E
Link: https://lore.kernel.org/r/20210530164923.18134-6-paul@crapouillou.net
Signed-off-by: Stephen Boyd <sboyd@kernel.org>