ARM: OMAP2+: GPMC: Support Software ECC scheme via DT

ARM: OMAP2+: GPMC: Support Software ECC scheme via DT

For v3.14 and prior, 1-bit Hamming code ECC via software was the
default choice for some boards e.g. 3430sdp.
Commit 9408e50680c7 in v3.15 changed the behaviour
to use 1-bit Hamming code via Hardware using a different ECC layout
i.e. (ROM code layout) than what is used by software ECC.

This ECC layout change causes NAND filesystems created in v3.14
and prior to be unusable in v3.15 and later. So don't mark "sw" scheme
as deperecated and support it.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>