platform: kendryte/k210: Add some padding for FDT fixups

The Kendryte K210 platform has built-in DTB and does not provides
FW_PAYLOAD_FDT_ADDR. This means the FDT fixups will be done on
the built-in DTB in absence of FW_PAYLOAD_FDT_ADDR. This patch
adds some padding in built-in DTB for FDT fixups.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>