powerpc/time: Rename mftbl() to mftb()

On PPC64, we have mftb().
On PPC32, we have mftbl() and an #define mftb() mftbl().

mftb() and mftbl() are equivalent, their purpose is to read the
content of SPRN_TRBL, as returned by 'mftb' simplified instruction.

binutils seems to define 'mftbl' instruction as an equivalent
of 'mftb'.

However in both 32 bits and 64 bits documentation, only 'mftb' is
defined, and when performing a disassembly with objdump, the displayed
instruction is 'mftb'

No need to have two ways to do the same thing with different
names, rename mftbl() to have only mftb().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/94dc68d3d9ef9eb549796d4b938b6ba0305a049b.1601556145.git.christophe.leroy@csgroup.eu