sd: sdhci: Implement basic vendor specific register support

sd: sdhci: Implement basic vendor specific register support

The Linux kernel's IMX code now uses vendor specific commands.
This results in endless warnings when booting the Linux kernel.

sdhci-esdhc-imx 2194000.usdhc: esdhc_wait_for_card_clock_gate_off:
	card clock still not gate off in 100us!.

Implement support for the vendor specific command implemented in IMX hardware
to be able to avoid this warning.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Message-id: 20200603145258.195920-2-linux@roeck-us.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>