clk: rockchip: simplify GRF handling in pll clocks

With the previous commit, the clock drivers now know at init time if the
GRF regmap is available. That means if it isn't available then, it also
won't become available later and we can therefore switch PLLs, that need
the GRF for the lock-status, to read-only mode - similar behaviour as the
aborting of rate changes we did before.

This saves some conditionals on every rate change and we can also drop
the rockchip_clk_get_grf function completely.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>