Documentation: bindings: add more configuration for rockchip emmc phy

Documentation: bindings: add more configuration for rockchip emmc phy

This patch add some optional configuration for dt. freq-sel can be used
to decide the phy sample clk in order to match the real freq of emmc
controller. dr-sel can be configured to match the requirement of different
drive strength of phy IO. opdelay should be used to adjust the output
delay for clk IO and data IO, which is useful for sloving timing issue.

Change-Id: I0b4da111581c76fbb96b15cd6be653aaa4843c33
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>