dt-bindings: display: samsung,exynos-mixer: Fix 'regs' typo

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

Fixes: 68e89bb3

 ("dt-bindings: display: samsung,exynos-mixer: convert to dtschema")
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>