char: hw_random: atmel: make it selectable for ARCH_MICROCHIP

LAN969x uses the Atmel HWRNG, 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: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>