arm64: Add config for Microchip SoC platforms

arm64: Add config for Microchip SoC platforms

Currently, Microchip SparX-5 SoC is supported and it has its own symbol.

However, this means that new Microchip platforms that share drivers need
to constantly keep updating depends on various drivers.

So, to try and reduce this lets add ARCH_MICROCHIP symbol that drivers
could instead depend on.

LAN969x is being worked on and it will be added under ARCH_MICROCHIP.

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>