docs: arm64: booting.rst: get rid of some warnings

docs: arm64: booting.rst: get rid of some warnings

Get rid of those warnings:

    Documentation/arm64/booting.rst:253: WARNING: Unexpected indentation.
    Documentation/arm64/booting.rst:259: WARNING: Block quote ends without a blank line; unexpected unindent.

By adding an extra blank lines where needed.

While here, use list markups on some places, as otherwise Sphinx
will consider the next lines as continuation of the privious ones.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/121b267be0a102fde73498c31792e5a9309013cc.1586881715.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>