MFD: allow cs5535-mfd to build on X86 only

Stephen ran into the following build error:

  drivers/mfd/cs5535-mfd.c:30:22: error: asm/olpc.h: No such file or directory

olpc.h exists only on x86 (and in the future, ARM).  Rather than
wrapping the include in an #ifdef, just change cs5535-mfd to only build
on x86.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>