ARM: OMAP2+: gpmc: Add device tree documentation for elm handle

In case ELM module available, omap2 NAND driver can opt for hardware
correction method for bit flip errors in NAND flash with BCH. Hence the
detection of ELM module is done through devicetree population of elm_id.
This patch update device tree documentation for gpmc-nand for elm-id
data population.

Signed-off-by: Philip Avinash <avinashphilip@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>