dt: fix reference to olpc,xo1.75-ec.txt

This file was converted and renamed.

Fixes: 7882d822 ("dt-bindings: spi: Convert spi-pxa2xx to json-schema")
Reviewed-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d0262854582ee754e4b8bd80677d96b3e098ea5c.1592203542.git.mchehab+huawei@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/misc/olpc,xo1.75-ec.txt

@@ -8,7 +8,7 @@ The embedded controller requires the SPI controller driver to signal readiness
 to receive a transfer (that is, when TX FIFO contains the response data) by
 strobing the ACK pin with the ready signal. See the "ready-gpios" property of the
 SSP binding as documented in:
-<Documentation/devicetree/bindings/spi/spi-pxa2xx.txt>.
+<Documentation/devicetree/bindings/spi/marvell,mmp2-ssp.yaml>.
 
 Example:
 	&ssp3 {