mtd: gpmi-nand: define ecc.strength

Fix an issue which was introduced by the recent addition of ecc.strength.

The ecc.strength wasn't set in gpmi-nand, resulting in the following crash:
[    2.550000] kernel BUG at drivers/mtd/nand/nand_base.c:3347!
...
[    2.550000] [<c020841c>] (nand_scan_tail+0x328/0x650) from [<c02f68e0>] (gpmi_nand_probe+0x43c/0x5a4)
[    2.550000] [<c02f68e0>] (gpmi_nand_probe+0x43c/0x5a4) from [<c01f6618>] (platform_drv_probe+0x14/0x18)
[    2.550000] [<c01f6618>] (platform_drv_probe+0x14/0x18) from [<c01f55b0>] (driver_probe_device+0x74/0x1fc)
[    2.550000] [<c01f55b0>] (driver_probe_device+0x74/0x1fc) from [<c01f57cc>] (__driver_attach+0x94/0x98)
[    2.550000] [<c01f57cc>] (__driver_attach+0x94/0x98) from [<c01f3d40>] (bus_for_each_dev+0x50/0x80)
[    2.550000] [<c01f3d40>] (bus_for_each_dev+0x50/0x80) from [<c01f4e18>] (bus_add_driver+0x188/0x25c)
[    2.550000] [<c01f4e18>] (bus_add_driver+0x188/0x25c) from [<c01f5a70>] (driver_register+0x78/0x138)
[    2.550000] [<c01f5a70>] (driver_register+0x78/0x138) from [<c043dc7c>] (gpmi_nand_init+0xc/0x30)
[    2.550000] [<c043dc7c>] (gpmi_nand_init+0xc/0x30) from [<c0008824>] (do_one_initcall+0x108/0x17c)
[    2.550000] [<c0008824>] (do_one_initcall+0x108/0x17c) from [<c042a8b8>] (kernel_init+0xfc/0x1bc)
[    2.550000] [<c042a8b8>] (kernel_init+0xfc/0x1bc) from [<c000fab4>] (kernel_thread_exit+0x0/0x8)

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>