devicetree: phy: rockchip-emmc add output-tapdelay-select

Update the rockchip-emmc-phy.txt and add the u32 property
'output-tapdelay-select'. This allow to set the otapdlysec register.

Tested with our customized rk3399 board to tune eMMC.
Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Link: https://lore.kernel.org/r/20201202082507.3536-3-chris.ruehl@gtsys.com.hkSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/phy/rockchip-emmc-phy.txt

@@ -18,6 +18,9 @@ Optional properties:
                         If not set, the default value of 50 will be applied.
  - enable-strobe-pulldown: Enable internal pull-down for the strobe line.
                            If not set, pull-down is not used.
+ - output-tapdelay-select: Specifies the phyctrl_otapdlysec register.
+                           If not set, the register defaults to 0x4.
+                           Maximum value 0xf.
 
 Example: