rockchip: sdhci: rk3568: bypass DLL when clk <= 52 MHz

For Rockchip platform, DLL bypass bit and start bit need to be set if
DLL is not locked.

With this change applied eMMC in my NanoPi R5S can run at 52 MHz.

Based on linux commit b75a52b0dda3 ("mmc: sdhci-of-dwcmshc: Update DLL
and pre-change delay for rockchip platform")

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>