sdhci_xenon: remove redundant code in property parsing

Remove redundant ofw property parsing in driver code, is already
taken care of in mmc_fdt_helpers.

Move ofw parsing to attach method.

Reviewed by: manu
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D31409

(cherry picked from commit 6ad816a999171874c36c73d5794d0d25ff8fd311)