pci: imx: Add iMX6SX compatible

The driver works fine with iMX6SX, add the missing compatible string.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>