cmd: fdt: Map address returned from fdt get addr to sysmem

The address returned from 'fdt get addr' command must be mapped
into sysmem, as this is a working FDT. Access to this address
without mapping it would lead to crash e.g. in sandbox.

The following command triggers the crash:
"
./u-boot -Dc 'fdt addr $fdtcontroladdr ; fdt get addr var / compatible ; md $var'
"

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>