efi_loader: only reserve memory if fdt node enabled

Sub-nodes of /reserved-memory may be disabled. In this case we should not
reserve memory in the memory map.

Reported-by: Patrick DELAUNAY <patrick.delaunay@st.com>
Fixes: fef907b2 ("efi_loader: create reservations after
ft_board_setup")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>