clk: mmp2: Add support for PLL clock sources

clk: mmp2: Add support for PLL clock sources

The clk-of-mmp2 driver pretends that the clock outputs from the PLLs are
constant, but in fact they are configurable.

Add logic for obtaining the actual clock rates on MMP2 as well as MMP3.
There is no documentation for either SoC, but the "systemsetting" drivers
from Marvell GPL code dump provide some clue as far as MPMU registers on
MMP2 [1] and MMP3 [2] go.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/lkundrak/linux-mmp3-dell-ariel.git/tree/drivers/char/mmp2_systemsetting.c
[2] https://git.kernel.org/pub/scm/linux/kernel/git/lkundrak/linux-mmp3-dell-ariel.git/tree/drivers/char/mmp3_systemsetting.c

A separate commit will adjust the clk-of-mmp2 driver.

Tested on a MMP3-based Dell Wyse 3020 as well as MMP2-based OLPC
XO-1.75 laptop.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lkml.kernel.org/r/20200309194254.29009-5-lkundrak@v3.sk
Signed-off-by: Stephen Boyd <sboyd@kernel.org>