ahci_imx: depend on CONFIG_MFD_SYSCON

ahci_imx makes use of regmap but the dependency wasn't specified in
Kconfig leading build failures if CONFIG_AHCI_IMX is enabled but
CONFIG_MFD_SYSCON is not.  Add the Kconfig dependency.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>

drivers/ata/Kconfig

@@ -99,7 +99,7 @@ config SATA_AHCI_PLATFORM
 
 config AHCI_IMX
 	tristate "Freescale i.MX AHCI SATA support"
-	depends on SATA_AHCI_PLATFORM
+	depends on SATA_AHCI_PLATFORM && MFD_SYSCON
 	help
 	  This option enables support for the Freescale i.MX SoC's
 	  onboard AHCI SATA.