mfd: abx500-core: Allow driver to built if COMPILE_TEST is enabled

The driver only has runtime but no build time dependency with ARCH_U300 ||
ARCH_U8500 So it can be built for testing purposes if COMPILE_TEST option
is enabled.

This is useful to have more build coverage and make sure that the driver
is not affected by changes that could cause build regressions.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/Kconfig

@@ -967,7 +967,7 @@ config MFD_SMSC
 
 config ABX500_CORE
 	bool "ST-Ericsson ABX500 Mixed Signal Circuit register functions"
-	default y if ARCH_U300 || ARCH_U8500
+	default y if ARCH_U300 || ARCH_U8500 || COMPILE_TEST
 	help
 	  Say yes here if you have the ABX500 Mixed Signal IC family
 	  chips. This core driver expose register access functions.