rockchip: clk: rk3399: adapt MMC clk configuration to the updated RK3399 DTS

The clocking of the designware MMC controller in the upstream
(i.e. Linux) RK3399 has changed/does not match what the current DTS in
U-Boot uses: the first clock entry now is HCLK_SDMMC instead of
SCLK_SDMMC.

With the simple clock driver used for the RK3399, this needs a change
in the selector understood by the various case statements in the driver
to ensure that the driver still loads successfully.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>