ARM: shmobile: Make sh7372 cpuidle handling more straightforward

The sh7372 cpuidle code uses the same artificially designed routine
shmobile_cpuidle_enter() as the .enter() callback for all of its
cpuidle states.  However, shmobile_cpuidle_enter() calls a different
"enter" function for each state using an array of function pointers
populated by the sh7372 PM initialization code.  Moreover, the
states[] array of the shmobile cpuidle driver is populated by that
code as well, although in principle it just might have been filled
with static data.

All of that complexity goes away if the sh7372 cpuidle code is
allowed to define its own cpuidle driver structure that can be passed
for registration to the common shmobile cpuidle initialization
routine, so modify the code accordingly.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Magnus Damm <damm@opensource.se>