mfd: at91-usart: Do not use compatible to register child devices

There are no separate serial/SPI devices under the at91-usart MFD, but
actually the MFD is used as a serial or a SPI device. There are no DT
sub-nodes for the MFD children, so we should use platform device API to
register the drivers.

Suggested-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>