riscv: Fix serial0 alias path

We have all MMIO devices under "/smb" DT node so the serial0 alias
path should have "/smb" prefix.

Fixes: 7c9aac00 ("riscv: Generate FDT at runtime for Guest/VM")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20220815101325.477694-6-apatel@ventanamicro.com


Signed-off-by: Will Deacon <will@kernel.org>