uboot-imx

icorem6: Use drive strength macros

Use driver strength macros instead of hex numbers.
- IMX6DQ_DRIVE_STRENGTH - 0x30
- IMX6SDL_DRIVE_STRENGTH - 0x28

Cc: Stefano Babic <sbabic@denx.de>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>