platform: arm64: Move ec devices to an ec subdirectory

commit 363c8aea "platform: Add ARM64 platform directory" added a
subdirectory for arm64 platform devices, but claims that all such
devices must be 'EC like'.

The arm64 MPAM driver manages an MMIO interface that appears in memory
controllers, caches, IOMMU and connection points on the interconnect.
It doesn't fit into any existing subsystem.

It would be convenient to use this subdirectory for drivers for other
arm64 platform devices which aren't closely coupled to the architecture
code and don't fit into any existing subsystem.

Move the existing code and maintainer entries to be under
drivers/platform/arm64/ec. The MPAM driver will be added under
drivers/platform/arm64/mpam.

Signed-off-by: James Morse <james.morse@arm.com>