ARM: at91: select ARCH_MICROCHIP

Like with the ARM64 Microchip platforms, lets add a generic ARCH_MICROCHIP
symbol and select it so that drivers that are reused for multiple product
generation or lines, can just depend on it instead of adding each SoC
symbol as their dependencies.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://lore.kernel.org/r/20250813174720.540015-3-robert.marko@sartura.hr


Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>