MAINTAINERS: Improve the PowerPC machines section

MAINTAINERS: Improve the PowerPC machines section

Add some documentation files to the corresponding machine sections
and mention the machine names in the section titles where it is
not so obvious (e.g. that "taihu" is a 405 machine).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20220105104800.407570-1-thuth@redhat.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>