docs: arm64: Fix indentation and doc formatting

Sphinx generates the following warnings for the arm64 doc
pages:

Documentation/arm64/memory.rst:158: WARNING: Unexpected indentation.
Documentation/arm64/memory.rst:162: WARNING: Unexpected indentation.

These indentations warnings can be resolved by utilising code
hightlighting instead.

Signed-off-by: Adam Zerella <adam.zerella@gmail.com>
Signed-off-by: Will Deacon <will@kernel.org>