arm: spear: Add command to switch between 1-bit HW ECC and SW BCH4

This patch adds the "nandecc" command to switch between the SPEAr600 internal
1-bit HW ECC and the 4-bit SW BCH4 ECC. This can be needed to support NAND
chips with a stronger ECC than 1-bit, as on the x600. And to dynamically
switch between both ECC schemes for backwards compatibility.

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