hw/riscv: boot: Reduce FDT address alignment constraints

hw/riscv: boot: Reduce FDT address alignment constraints

We previously stored the device tree at a 16MB alignment from the end of
memory (or 3GB). This means we need at least 16MB of memory to be able
to do this. We don't actually need the FDT to be 16MB aligned, so let's
drop it down to 2MB so that we can support systems with less memory,
while also allowing FDT size expansion.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/992
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Message-Id: <20220608062015.317894-1-alistair.francis@opensource.wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>