mmc: xenon_sdhci: Add missing host->max_clk to Xenon SDHCI driver

The Xenon SDHCI driver just missed the integration of this patch:

git ID 6d0e34bf
mmc: sdhci: Distinguish between base clock and maximum peripheral frequency

With this patch applied, the SDHCI subsystem complains now with this warning
while probing:

sdhci_setup_cfg: Hardware doesn't specify base clock frequency

This patch fixes this issue, by providing the missing host->max_clk
variable to the SDHCI subsystem.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Hu Ziji <huziji@marvell.com>
Cc: Victor Gu <xigu@marvell.com>
Cc: Konstantin Porotchkin <kostap@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>