serial: pl01x: expose skip_init platdata option in DT

To be able to represent the skip-init platdata element with OF_CONTROL,
it needs to be read from the device tree as well and put into the platform data.

Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Fabian Vogt <fvogt@suse.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/device-tree-bindings/serial/pl01x.txt

@@ -5,3 +5,6 @@ Required properties:
 - reg: exactly one register range with length 0x1000
 - clock: input clock frequency for the UART (used to calculate the baud
   rate divisor)
+
+Optional properties:
+- skip-init: if present, the baud rate divisor is not changed