mfd: Kconfig: ABX500_CORE should depend on ARCH_U8500

The ST-Ericsson ABX500 Mixed Signal IC family chips are only present on
ST-Ericsson U8500 Series platforms.  Hence add a dependency on
ARCH_U8500, to prevent asking the user about this driver when
configuring a kernel without U8500 support.

Also, merely enabling CONFIG_COMPILE_TEST should not enable additional
code, and thus should not enable this driver by default.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>