mmc: sdhci-esdhc-imx: no fail when no pinctrl available

When using jailhouse to support two Linux on i.MX8MQ EVK, we use the
1st Linux to configure pinctrl for the 2nd Linux. Then the 2nd Linux
could use the mmc without pinctrl driver.

So give a warning message when no pinctrl available, but no fail probe.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/1582100563-20555-3-git-send-email-haibo.chen@nxp.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>