RISC-V: Add CONFIG_HVC_RISCV_SBI=y to defconfig

The SBI exists on all RISC-V systems, so there's no reason not to
compile this driver in.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/configs/defconfig

@@ -44,6 +44,7 @@ CONFIG_INPUT_MOUSEDEV=y
 CONFIG_SERIAL_8250=y
 CONFIG_SERIAL_8250_CONSOLE=y
 CONFIG_SERIAL_OF_PLATFORM=y
+CONFIG_HVC_RISCV_SBI=y
 # CONFIG_PTP_1588_CLOCK is not set
 CONFIG_DRM=y
 CONFIG_DRM_RADEON=y