arm64: update ARCH_MVEBU for Marvell Armada 7K/8K support

This commit updates the ARCH_MVEBU Kconfig option introduced for
Armada 3700 to also be used for the Armada 7K and 8K platforms, by:

 - Selecting the appropriate clock and irqchip drivers
 - Updating the help text to mention Armada 7K and 8K

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>