arm64: dts: rockchip: remove keep-power-in-suspend from sdhci of rk3399-sapphire

sdhci for rk3399-sapphire works for eMMC but keep-power-in-suspend
is an optional property for SDIO.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>