ARM: dts: imx7d: Correct speed grading fuse settings

The 800MHz opp speed grading fuse mask should be 0xd instead
of 0xf according to fuse map definition:

SPEED_GRADING[1:0]	MHz
	00		800
	01		500
	10		1000
	11		1200

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>