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>