sdhci: fdt: Correctly export clock per the binding

The binding says that we can have one or two clocks to export.
The first one is the actual sdclock while the second is the sample clock.
Both have the same parent, clk_xin.
Correctly export the clocks for RK3399 and ZynqMP.
No need to use a high ID as before, we have our own clock domain so use
ids starting at 1 as all exported clocks should be.

Reviewed-by: bz
Differential Revision: https://reviews.freebsd.org/D41810
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 81a4fe38a6ce818bb7cba548bb2c697429fa9479)