dt-binding: fsl-lpuart: use exact SoC revision to document binding

use exact SoC revision instead of wildcard describing
to make the binding more clearer.
Signed-off-by: NJingchang Lu <jingchang.lu@freescale.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/fsl-lpuart.txt

 * Freescale low power universal asynchronous receiver/transmitter (lpuart)
 
 Required properties:
-- compatible : Should be "fsl,<soc>-lpuart"
+- compatible :
+  - "fsl,vf610-lpuart" for lpuart compatible with the one integrated
+    on Vybrid vf610 SoC with 8-bit register organization
+  - "fsl,ls1021a-lpuart" for lpuart compatible with the one integrated
+    on LS1021A SoC with 32-bit big-endian register organization
 - reg : Address and length of the register set for the device
 - interrupts : Should contain uart interrupt
 - clocks : phandle + clock specifier pairs, one for each entry in clock-names