powerpc: Cleanup idle for e500

e500 idle setup is a bit messy.

e500_idle() is used for PPC32 while book3e_idle() is used for PPC64.
As they are mutually exclusive, call them all e500_idle().

Use CONFIG_MPC_85xx instead of PPC32 + E500 in Makefile and rename
idle_e500.c to idle_85xx.c .

Rename idle_book3e.c to idle_64e.c and remove #ifdef PPC64 in
as it's only built on PPC64.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8039301334e948974c85ec5ef2db37751075185b.1663606876.git.christophe.leroy@csgroup.eu