ARM: imx: imx8m: Fix board_get_usable_ram_top()

The 4 GiB boundary is at 0xffffffff+1 , not at 0x80000000, fix this.

The PHYS_SDRAM of i.MX8M is at 0x40000000 , so to restrict ram_top
below 4 GiB, the ram_top has to be set to 0xffffffff as it is not
an offset from the start of PHYS_SDRAM, but rather a physical address
marking the topmost allowed DRAM address.

Fixes: e27bddff

 ("imx8m: Restrict usable memory to space below 4G boundary")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Frieder Schrempf <frieder.schrempf@kontron.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>