powerpc/8xx: Reduce alignment constraint for kernel memory

8xx has three large page sizes: 8M, 512k and 16k.

A too big alignment can lead to wasting memory. On a board which has
only 32 MBytes of RAM, every single byte is worth it and a 512k
alignment is sometimes too much.

Allow mapping kernel memory with 16k pages and reduce the constraint
on kernel memory alignment. 512k and 16k pages are handled the same
way so reverse tests in order to make 8M pages the special case and
other ones (512k and 16k) the alternative.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/fa9927b70df13627cdf10b992ea71d6562c7760e.1746191262.git.christophe.leroy@csgroup.eu