rockchip: Fix spl mmc boot device ofpath

Linux v5.7-rc1 dts(i) sync has changed the sdmmc node from
dwmmc@fe320000 to mmc@fe320000 and this ofpath is being
used in rockchip spl bootdevice code.

So, update the ofpath with a new node name and prefix "same-as-spl"
to missing u-boot,spl-boot-order.

Bug log:
U-Boot SPL 2020.07-rc2-00256-g9c5fef5774 (May 24 2020 - 20:20:43 +0530)
Trying to boot from MMC2
mmc_load_image_raw_sector: mmc block read error
Trying to boot from MMC1
mmc_load_image_raw_sector: mmc block read error
SPL: failed to boot from all boot devices

Fixes: 167efc2c ("arm64: dts: rk3399: Sync v5.7-rc1 from Linux"
Signed-off-by: Suniel Mahesh <sunil@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>