I added some debugging on top of your patch, and get:
platform basic-mmio-gpio.1.auto: Device node exists
[/smb/motherboard/iofpga@7,00000000/sysreg@00000/sys_led@08],
of_driver_match_device() failed
platform basic-mmio-gpio.1.auto: platform_match_id() succeeded
platform basic-mmio-gpio.2.auto: Device node exists
[/smb/motherboard/iofpga@7,00000000/sysreg@00000/sys_mci@48],
of_driver_match_device() failed
platform basic-mmio-gpio.2.auto: platform_match_id() succeeded
platform basic-mmio-gpio.3.auto: Device node exists
[/smb/motherboard/iofpga@7,00000000/sysreg@00000/sys_flash@4c],
of_driver_match_device() failed
platform basic-mmio-gpio.3.auto: platform_match_id() succeeded
So it isn't the mmc driver failing to instantiate directly,
but (I think) vexpress-sysreg.