tty: serial: atmel: make it selectable for ARCH_MICROCHIP
LAN969x uses the Atmel USART serial, so make it selectable for
ARCH_MICROCHIP to avoid needing to update depends in future if other
Microchip SoC-s use it as well.
Signed-off-by: Robert Marko <robert.marko@sartura.hr> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Acked-by: Daniel Machon <daniel.machon@microchip.com> Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>