powerpc: Make generic_calibrate_decr() the default

ppc_md.calibrate_decr() is a mandatory item. Its nullity is never
checked so it must be non null on all platforms.

Most platforms define generic_calibrate_decr() as their
ppc_md.calibrate_decr(). Have time_init() call
generic_calibrate_decr() when ppc_md.calibrate_decr() is NULL,
and remove default assignment from all machines.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/6cb9865d916231c38401ba34ad1a98c249fae135.1676711562.git.christophe.leroy@csgroup.eu