dt-bindings: mfd: add at91-usart.h from Linux

Copy include file dt-bindings/mfd/at91-usart.h from Linux

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
1 file changed