spi: cadence_qspi: Change to use devfdt_get_addr_index()

Change to use devfdt_get_addr_index() function to get fdt address.

Original code has compilation warning below:

drivers/spi/cadence_qspi.c: In function ‘cadence_spi_ofdata_to_platdata’:
drivers/spi/cadence_qspi.c:297:18: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  plat->regbase = (void *)data[0];
                  ^
drivers/spi/cadence_qspi.c:298:18: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  plat->ahbbase = (void *)data[2];
                  ^
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>