rockchip: rk3568: Fix boot device detection

The boot source node path for emmc is using the old sdhci name.
Replace with correct mmc name and also add same-as-spl to boot order.

Fixes: 0d61f8e5 ("rockchip: rk3568: add boot device detection")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>