dt-bindings: mmc: xenon: add AP807 compatible string

This patch adds DT binding description for the Xenon
SD/MMC controller included in AP807 north bridge.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Link: https://lore.kernel.org/r/20210112094655.12274-2-kostap@marvell.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>