Documentation: arm64: fix amu.rst doc warnings

Fix bullet list formatting to eliminate doc warnings:

Documentation/arm64/amu.rst:26: WARNING: Unexpected indentation.
Documentation/arm64/amu.rst:60: WARNING: Unexpected indentation.
Documentation/arm64/amu.rst:81: WARNING: Unexpected indentation.
Documentation/arm64/amu.rst:108: WARNING: Unexpected indentation.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Ionela Voinescu <ionela.voinescu@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>