mmc: meson-mx-sdio: Add a driver for the Amlogic Meson8 and Meson8b SoCs

Add a driver for the SDIO/MMC host found on the Amlogic Meson SoCs. This
is an MMC controller which provides an interface between the application
processor and various memory cards. It supports the SD specification
v2.0 and the eMMC specification v4.41.

The controller provides an internal "mux" which allows connecting up to
three MMC devices to it. Only one device can be used at a time though
since the registers are shared across all devices. The driver takes care
of synchronizing access (similar to the dw_mmc driver).
The maximum supported bus-width is 4-bits.

Amlogic's GPL kernel sources call the corresponding driver "aml_sdio" to
differentiate it from the other MMC controller in (at least the Meson8
and Meson8b) the SoCs (they call the other drivers aml_sdhc and
aml_sdhc_m8, which seem to support a bus-width of up to 8-bits). This
means that there are three different MMC host controller IP blocks from
Amlogic (each of them with completely own register layout and features):
- "SDIO": 1 and 4 bit bus width, support for high-speed modes up to
  UHS-I SDR50, part of Meson6, Meson8 and Meson8b (the driver from this
  patch targets this controller)
- "SDHC": 1, 4 and 8 bit bus width, compatible with standard iNAND
  interface, support for speeds up to HS200 and MMC spec up to version
  4.5x, part of Meson8 and Meson8b SoCs (there is no mainline driver
  for this controller yet)
- "SDEMMC": 1, 4 and 8 bit bus width, support for speeds up to HS400
  and MMC spec up to version 5.0, part of the Meson GX (64-bit) SoCs
  (supported by the meson-gx MMC host driver)

Signed-off-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>