powerpc/8xx: Reduce default size of module/execmem area

8xx boards don't have much memory, the two I know have respectively
32Mbytes and 128Mbytes, so there is no point in having 256 Mbytes of
memory for module text.

Reduce it to 32Mbytes for 8xx, that's more than enough.

Nevertheless, make it a configurable value so that it can be customised
if needed.

Also add a build verification for overlap of module execmem space
with user PMD.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/8db23b61e33a0d1913d814f94bfe71ba7ac78b0f.1724173828.git.christophe.leroy@csgroup.eu