clk: mmp2: Rename mmp2_pll_init() to mmp2_main_clk_init()

This is a trivial rename for a routine that registers more clock sources
than the PLLs -- there's also a XO.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lkml.kernel.org/r/20200519224151.2074597-7-lkundrak@v3.sk


Signed-off-by: Stephen Boyd <sboyd@kernel.org>