ARM: imx8m: verdin-imx8mm: Increase bootm size to 64 MiB

Uncompressed aarch64 kernel Image are rather large, increase the bootm
size limit to 64 MiB to cater for that.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Cc: Max Krummenacher <max.krummenacher@toradex.com>
Cc: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>