dt-bindings: mmc: sdhci-pxa: Fix 'regs' typo

The correct property name is 'reg' not 'regs'.

Fixes: ae5c0585

 ("dt-bindings: mmc: Convert sdhci-pxa to json-schema")
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231213224219.2191721-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>