mtd: Use default mtdparts/mtids when not defined in the environment

U-boot provides a mean to define default values for mtdids and mtdparts
when they're not defined in the environment. Patch mtd_probe_devices()
to use those default values when env_get("mtdparts") or
env_get("mtdids") return NULL.

This implementation is based on the logic found in cmd/mtdparts.c.

Fixes: 5db66b3a

 ("cmd: mtd: add 'mtd' command")
Reported-by: Stefan Roese <sr@denx.de>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Stefan Roese <sr@denx.de>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>