ARM: dts: keystone: Add "ti,da830-uart" compatible string

The TI Keystone SoCs have extra UART registers beyond the standard 8250
registers, so we need a new compatible string to indicate this. Also, at
least one of these registers uses the full 32 bits, so we need to specify
reg-io-width in addition to reg-shift.

"ns16550a" is left in the compatible specification since it does work as
long as the bootloader configures the SoC UART power management registers.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>