gxt4500: enable on non-PPC architectures

These chips can be present at least on x86 too - Fire GL2 AGP has GXT6000P but
this driver is currently limited to PPC.
Enable it for all architectures and add chip configuration for little-endian.

Tested on x86 with Fire GL2 AGP.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>