powerpc/fixmap: don't clear fixmap area in paging_init()

fixmap is intended to map things permanently like the IMMR region on
FSL SOC (8xx, 83xx, ...), so don't clear it when initialising paging()

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/41c99bc06394a6bc2888631cb98a3ed2ae281ddb.1568295907.git.christophe.leroy@c-s.fr