MLK-10657: mtd: NAND: correct bitflip for erased NAND page

i.MX6QP and i.MX7D BCH module integrated a new feature to detect the
bitflip number for erased NAND page. So for these two platform, set the
erase threshold to gf/2 and if bitflip detected, GPMI driver will
correct the data to all 0xFF.

Also updated the imx6qp dts file to ditinguish the GPMI module for i.MX6Q
with the one for i.MX6QP.

Signed-off-by: Han Xu <b45815@freescale.com>
(cherry picked from commit: 4302ab74a301626e7e0b9cb398a23b2e488cfa6b)