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.org


Signed-off-by: Rob Herring <robh@kernel.org>