powerpc/time: Make mftb() common to PPC32 and PPC64

No need to have two versions that are identical.

CONFIG_PPC_CELL is only selected by PPC64 targets.
CONFIG_E500 is the only PPC64 target selecting CONFIG_FSL_BOOK3E.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6bf23ec744aab4ba63506a011f6a145ea35d620d.1601556145.git.christophe.leroy@csgroup.eu