Add card_busy host ops in sdhci_omap to check card busy status.

The voltage switching sequence for AM572x platform is mentioned
in Figure 25-48. eMMC/SD/SDIO Power Switching Procedure of
AM572x Sitara Processors Silicon Revision 2.0, 1.1 TRM
(SPRUHZ6I - October 2014–Revised April 2017 [1]).

In the voltage switching sequence, CLKEXTFREE bit in MMCHS_CON
should also be set after switching to 1.8v which is also taken
care in the card_busy ops.

[1] -> http://www.ti.com/lit/ug/spruhz6i/spruhz6i.pdfSigned-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>