Build the driver for the in-SoC AHCI SATA host as module, just like for
the other subtargets. No board requires booting off SATA, so we don't
need to have it built-in. All boards with SATA support already select
kmod-ata-ahci-mtk which provides the module.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>