MAINTAINERS: arm64: Remove boot/dts/ directory from arm64 entry

The arm-soc tree does a good job handling .dts files, so exclude them
from the ARM64 entry in MAINTAINERS.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Will Deacon <will.deacon@arm.com>