ARM: dts: am437x-gp-evm: Use BCH16 ECC scheme instead of BCH8

am437x-gp-evm uses a NAND chip with page size 4096 bytes
and spare area of 225 bytes per page.

For such a setup it is preferrable to use BCH16 ECC scheme over
BCH8. This also makes it compatible with ROM code ECC scheme so
we can boot with NAND after flashing from kernel.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Pekon Gupta <pekon@pek-sem.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>