Platform: OLPC: Require CONFIG_POWER_SUPPLY for XO-1.75 EC

ERROR: "power_supply_put" [drivers/platform/olpc/olpc-xo175-ec.ko] undefined!
  ERROR: "power_supply_changed" [drivers/platform/olpc/olpc-xo175-ec.ko] undefined!
  ERROR: "power_supply_get_by_name" [drivers/platform/olpc/olpc-xo175-ec.ko] undefined!

Adding the dependency seems like a more reasonable thing compared to
ifdef-ing the bits, as if one has an XO-1.75 they almost certainly want
a baterry and AC adapter support.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>