imx8m: Restrict usable memory to space below 4G boundary

Some IPs have their accessible address space restricted by the
interconnect. Let's make sure U-Boot only ever uses the space below
the 4G address boundary (which is 3GiB big), even when the effective
available memory is bigger.

We implement board_get_usable_ram_top() for all i.MX8M SoCs, as the
whole family is affected by this.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>