serial: vt8500: Add missing binding document for arch-vt8500 serial driver.

The binding document for the vt8500/wm8xxx SoC UART driver is missing.
This patch adds the binding document.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Rob Herring <robh@kernel.org>
1 file changed