x86/olpc: Fix build error with CONFIG_MFD_CS5535=m

When building a 32-bit config which has the above MFD item as module
but OLPC_XO1_PM is enabled =y - which is bool, btw - the kernel fails
building with:

  ld: arch/x86/platform/olpc/olpc-xo1-pm.o: in function `xo1_pm_remove':
  /home/boris/kernel/linux/arch/x86/platform/olpc/olpc-xo1-pm.c:159: undefined reference to `mfd_cell_disable'
  ld: arch/x86/platform/olpc/olpc-xo1-pm.o: in function `xo1_pm_probe':
  /home/boris/kernel/linux/arch/x86/platform/olpc/olpc-xo1-pm.c:133: undefined reference to `mfd_cell_enable'
  make: *** [Makefile:1030: vmlinux] Error 1

Force MFD_CS5535 to y if OLPC_XO1_PM is enabled.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Lubomir Rintel <lkundrak@v3.sk>
Cc: x86@kernel.org
Link: http://lkml.kernel.org/r/20181005131750.GA5366@zn.tnic