powerpc/devicetrees: Change 'gpios' to 'cs-gpios' on fsl, spi nodes

Since commit 0f0581b2 ("spi: fsl: Convert to use CS GPIO
descriptors"), the prefered way to define chipselect GPIOs is using
'cs-gpios' property instead of the legacy 'gpios' property.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7556683b57d8ce100855857f03d1cd3d2903d045.1574943062.git.christophe.leroy@c-s.fr