fdtdec: Fix boundary check

fdtdec: Fix boundary check

In U-Boot, the reserved memory end address is considered as a inclusive
address. This notion is followed while adding a reserved memory node to
the DT.

For example:
end_address = start_address + size - 1

Follow the same notion and fix the end address computation while checking
for existing nodes.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>