arm64: dts: renesas: white-hawk-cpu: Add eMMC support

Describe the eMMC on the White Hawk CPU board.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/253cd479e55486dd0d3e3804add405e41c32d53b.1665558371.git.geert+renesas@glider.be