sdhci_xenon: add AP807 compatible string

This patch adds compatible string for xenon controller found on
AP807 north brige. It is fully compatible with existing driver.

Reviewed by: manu
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D31407

(cherry picked from commit 1b9ce0885e5f53ea5818ee593babf11f6b5d5761)