imx: imx8qm: enable relocation of fdt and initrd

Set CONFIG_SYS_BOOTMAPSZ to the amount of memory available which is needed
to relocate the kernel, device tree and initrd.

Remove 'fdt_high' and 'initrd_high' environment variables from default
environment which prevents relocation of FDT and initrd.

Signed-off-by: Oliver Graute <oliver.graute@kococonnector.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>