mailbox: exynos: Drop unused register definitions

mailbox: exynos: Drop unused register definitions

Leaving these dead definitions in place hides which registers are
actually being used by the hardware, making the driver harder to read
and maintain. Remove them to clean up the file.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Alexey Klimov <alexey.klimov@linaro.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>