mmc: zynq_sdhci: Allow configuring zero Tap values

Allow configuring ITAP and OTAP values with zero to avoid failures in
some cases (one of them is SD boot mode). Legacy, SDR12 modes require
to program the ITAP and OTAP values as zero, whereas for SDR50 and SDR104
modes ITAP value is zero.

In SD boot mode firmware configures the SD ITAP and OTAP values and
in this case u-boot has to re-configure required tap values(including zero)
based on the operating mode.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>