ARM: OMAP4: MMC: increase delay for pbias

4 micro seconds is not enough for PBIAS if MMC regulator is
enabled from MMC regulator OFF.
Increase the delay for PBIAS to stabilize.
Wait for PBIAS and timeout if not.

Resolves MMC/SD failure on OMAP4
"Pbias Voltage is not same as LDO"

Signed-off-by: Balaji T K <balajitk@ti.com>
Acked-by:  Kishore Kadiyala <kishore.kadiyala@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>