Thers are only two parent PLLs that APLL and GPLL for core on the
previous SoCs (RK3066/RK3188/RK3288/RK3368). Hence, we set fixed
GPLL as alternate parent when core is switching freq.

Since RK3399 big.LITTLE architecture, we need to select and adapt
more PLLs (ALPLL/ABPLL/DPLL/GPLL) sources.
Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>