ahci: imx: PLL clock needs 100us to settle down

The commit e783c51c (ahci: imx: software workaround for phy reset issue
in resume) calls imx_sata_phy_reset() to reset phy immediately after
SATA MPLL is enabled.  It seems working fine mostly, but fails in some
case as below.

...
ahci-imx 2200000.sata: failed to reset phy: -110
ahci-imx: probe of 2200000.sata failed with error -110

After talking to the designer, we learnt that when enabling i.MX6Q SATA
MPLL, we need to wait 100us for it to settle down for safety.  Add this
required delay to fix above failure.
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/ata/ahci_imx.c

@@ -258,6 +258,8 @@ static int imx_sata_enable(struct ahci_host_priv *hpriv)
 				   IMX6Q_GPR13_SATA_MPLL_CLK_EN,
 				   IMX6Q_GPR13_SATA_MPLL_CLK_EN);
 
+		usleep_range(100, 200);
+
 		ret = imx_sata_phy_reset(hpriv);
 		if (ret) {
 			dev_err(dev, "failed to reset phy: %d\n", ret);