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>
diff --git a/board/emulation/qemu-riscv/Kconfig b/board/emulation/qemu-riscv/Kconfig
index ed005e5..0d865ac 100644
--- a/board/emulation/qemu-riscv/Kconfig
+++ b/board/emulation/qemu-riscv/Kconfig
@@ -33,5 +33,6 @@
 	imply CMD_FAT
 	imply BOARD_LATE_INIT
 	imply OF_BOARD_SETUP
+	imply SIFIVE_SERIAL
 
 endif