staging: Make SystemBase PCI Multiport UART only for x86

The sb105x SystemBase PCI UART driver in staging has code specific for
x86. This breaks allyesconfig builds for other archs. For now, the quick
fix is simply to make this driver depend on x86.

When I get time, I'll swap this card out of my main machine and put it
into my PowerPC64 box, and get it working there. Then it may become a
good samaritan and play nice with other archs.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>