rockchip: rk3399: update PPLL and pmu_pclk frequency

Update PPLL to 676MHz and PMU_PCLK to 48MHz, because:
1. 48MHz can make sure the pwm can get exact 50% duty ratio, but 99MHz
can not,
2. We think 48MHz is fast enough for pmu pclk and it is lower power cost
than 99MHz,
3. PPLL 676 MHz and PMU_PCLK 48MHz are the clock rate we are using
internally for kernel,it suppose not to change the bus clock like pmu_pclk
in kernel, so we want to change it in uboot.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>