i2c: eg20t: Fix Kconfig dependencies

The i2c-eg20t 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. Add COMPILE_TEST as an alternative, so that the
driver can still be build-tested elsewhere.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>