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

mftbu() is always defined now, so the #ifdef can be removed
and replaced by an IS_ENABLED(CONFIG_PPC64) inside the
PPC32 version of get_tb().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/47e49717d2643169ffcbe5d507f184cf49f0fe95.1601556145.git.christophe.leroy@csgroup.eu