dt-bindings: Add binding for atmel-usart in SPI mode

This patch moves the bindings for serial from serial/atmel-usart.txt to
mfd/atmel-usart.txt and adds bindings for USART in SPI mode.

Signed-off-by: Radu Pirea <radu.pirea@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>