powerpc/vdso32: implement clock_getres entirely

clock_getres returns hrtimer_res for all clocks but coarse ones
for which it returns KTIME_LOW_RES.

return EINVAL for unknown clocks.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/37f94e47c91070b7606fb3ec3fe6fd2302a475a0.1575273217.git.christophe.leroy@c-s.fr