powerpc/mpic_timer: avoid struct timeval

In an effort to remove all instances of 'struct timeval'
from the kernel, I'm changing the powerpc mpic_timer interface
to use plain seconds instead. There is only one user of this
interface, and that doesn't use the microseconds portion, so
the code gets noticeably simpler in the process.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>