cmd_scsi: Enable SoC AHCI device on platforms with PCI

Current driver assumes the AHCI is connected to PCI, this is not
true on some SoCs, e.g. LS1021A, which has PCI but the AHCI is
in SoC. This patch will enable embedded AHCI devices on platforms
with PCI.
PCI AHCI devices still can be used by commenting CONFIG_SCSI_AHCI_PLAT
option in head file.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>