spi: dt-bindings: Fix 'reg' child node schema

The schema for SPI child nodes' 'reg' property is not complete. 'reg' is
a matrix of cells. The schema needs to define both the number of 'reg'
entries and constraints on each entry.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220126231326.1636199-1-robh@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>