net: pch_gbe depends on x86_32

The pch_gbe driver is for a companion chip to the Intel Atom E600
series processors. These are 32-bit x86 processors so the driver is
only needed on X86_32.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>