IP block found on several generations of i.MX family does not use
vanilla SDHCI implementation and it comes with a number of quirks.

Introduce i.MX SDHCI subtype of SDHCI block to add code necessary to
support unmodified Linux guest driver.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Marcel Apfelbaum <marcel.apfelbaum@zoho.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-devel@nongnu.org
Cc: qemu-arm@nongnu.org
Cc: yurovsky@gmail.com
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAndrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
[PMM: define and use ESDHC_UNDOCUMENTED_REG27]
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>