Platform: OLPC: Add a config menu category for XO 1.75

Randy Dunlap says:

  drivers/platform/olpc/Kconfig needs to use "menuconfig" like all of the
  other Kconfig files in drivers/platform/ so that its menu is listed in
  the correct place in *config interfaces.

Otherwise he's sad.

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>