porting-guide.rst: Fix some sections' level

porting-guide.rst: Fix some sections' level

Fix the level of the section
"13.   Function : plat_setup_psci_ops() [mandatory]",
including all the subsections.

Fix the level of the section
"12.7.   plat_psci_ops.pwr_domain_suspend_pwrdown_early() [optional]"
to lower it like the surrounding functions.

Change-Id: I781823bc96ece669f8fde4bd39c4e333c7bf4d1a
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>