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: NTejun Heo <tj@kernel.org>
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>