MLK-23583 doc: imx: habv4: Consolidate mx8m documentation

MLK-23583 doc: imx: habv4: Consolidate mx8m documentation

Old documentation styling mentioned all iMX8M devices variants (iMX8M,
iMX8MM, iMX8MN) for every SoC reference.

As the secure/encrypted boot procedure is similar for all the variants
(including iMX8MP), make this information common for the whole iMX8M
family to keep the documentation clean instead of adding iMX8MP on every
reference.
Specific information for single variants is described when needed.

Signed-off-by: Vanessa Maegima <vanessa.maegima@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 24c72869a608ffbcce908770953a6d87514f2253)
(cherry picked from commit bd591f59e2b8d25484873d03b61c464ca54e14cc)