uboot-imx

riscv: qemu: Imply SIFIVE_SERIAL for emulation

This patch enables SiFive UART driver for QEMU RISC-V emulation
by implying SIFIVE_SERIAL on BOARD_SPECIFIC_OPTIONS.

Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>