arm64: dts: cn913x-db: enable MMC HS400

This patch adds necessary flags in the device tree
which enable HS400 mode on AP807 MMC controller
on the CN913x-DB board.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>