MMC: Fix for capacity calculation on eMMC

  The current mmc driver returns erroneous capacity information for
  eMMC.  The capacity of eMMC devices is available only in the ext-CSD
  register.  This patch add code to read the ext-CDSD register and
  correctly calculate eMMC capacity.

Signed-off-by: Sukumar Ghorai <s-ghorai@ti.com>
Acked-by: Steve Sakoman <steve@sakoman.com>