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>