Name the Pin Function Controller subnode for MMC0 after its device name,
instead of using the generic and indexless "mmc".

This avoids conflicts when enabling support for more MMC interfaces
later, either here or in a DT overlay.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>