mfd: Update kconfig for ab8500 core driver

This patch add a dependancy for ab8500-core driver so as to depend on
u8500 platform.

This patch also fixes the build issues(powerpc_allyesconfig) for the
patch 03f582a93ecca6e9584b622570022abf08ed03ec (misc: Add ab8500 pwm
driver)

Signed-off-by: Arun Murthy <arun.murthy@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/Kconfig

@@ -475,7 +475,7 @@ config EZX_PCAP
 
 config AB8500_CORE
 	bool "ST-Ericsson AB8500 Mixed Signal Power Management chip"
-	depends on GENERIC_HARDIRQS && ABX500_CORE && SPI_MASTER
+	depends on GENERIC_HARDIRQS && ABX500_CORE && SPI_MASTER && ARCH_U8500
 	select MFD_CORE
 	help
 	  Select this option to enable access to AB8500 power management