arm: spear: Add BCH4 SW support to SPEAr600 x600 board

This board is equipped with a Micron NAND chip (MT29F1G08ABADAH4) that
needs 4-bit ECC. But the SPEAr600 only supports 1-bit HW ECC internally.
This patch enables the SW 4-bit BCH support for this board.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Viresh Kumar <viresh.kumar@linaro.org>