Documentation: bindings: add description of phy for sdhci-of-arasan

This patch adds phys and phy-names for sdhci-of-arasan as required
properties for arasan,sdhci-5.1, and details the example as well.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>