ath79: add pll for archer c7

ath79: add pll for archer c7

commit 4b9680f fixed pll settings and the correct pll set
by bootloader is overrided by value in qca9557.dtsi which
is incorrect for Archer C7 and breaks ethernet. Add pll
values for archer c7 to fix ethernet connection.

This individual pll tweak has been cherry picked from github pr 1260
which changes a couple of things in a single commit and should be
ideally split.  This commit get archer v7 back and working.

Tested: archer c7 v2

Original combined commit authored by:
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
c7 fix only split out by:
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>