DT: ARM: pxa: Remove incorrect binding from documentation

Remove "mrvl,lpss-ssp" property from documentation because LPSS SSP type is
for certain Intel platforms. I believe commit a6e56c28
("ARM: pxa: ssp: add DT bindings") added it by accident by copying all
enum pxa_ssp_type types from include/linux/pxa2xx_ssp.h.

Please note this was removed from arch/arm/plat-pxa/ssp.c by the
commit b692cb83 ("ARM: pxa: ssp: Fix build error by removing originally
incorrect DT binding").
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/serial/mrvl,pxa-ssp.txt

@@ -10,7 +10,6 @@ Required properties:
 				mvrl,pxa168-ssp
 				mrvl,pxa910-ssp
 				mrvl,ce4100-ssp
-				mrvl,lpss-ssp
 
 	- reg:		The memory base
 	- dmas:		Two dma phandles, one for rx, one for tx