powerpc: Remove CONFIG_PPC_BOOK3E_MMU

CONFIG_PPC_BOOK3E_MMU is redundant with CONFIG_PPC_E500.

Remove it.

Also rename mmu-book3e.h to mmu-e500.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c5549cd59a131204ff94ab909cad2e2dad4ddf2f.1663606876.git.christophe.leroy@csgroup.eu