mmc: sdhci: allow for eMMC 74 clock generation by controller

Snippet of code for how adaptation layer should handle the call:
/*
 * eMMC spec calls for the host to send 74 clocks to the card
 * during initialization, right after voltage stabilization.
 * create the clocks manually right here.
 */
void generate_init_clocks_A0(struct sdhci_host *host, u8 power_mode)
{
	struct sdhci_mmc_slot *slot = sdhci_priv(host);

	if (slot->power_mode == MMC_POWER_UP &&
	    power_mode == MMC_POWER_ON) {
		/* controller specific code here */
		/* slot->power_mode holds previous power setting */
	}
	slot->power_mode = power_mode;
}

Signed-off-by: Philip Rakity <prakity@marvell.com>
Signed-off-by: Chris Ball <cjb@laptop.org>