hw/riscv/Kconfig: Add missing dependency MICROCHIP_PFSOC -> SERIAL

hw/riscv/Kconfig: Add missing dependency MICROCHIP_PFSOC -> SERIAL

Commit a8fb0a500a6 ("hw/char: Add Microchip PolarFire SoC MMUART
emulation") added a dependency on the SERIAL model, but forgot to
add the Kconfig selector.
Add the dependency to the MCHP_PFSOC_MMUART symbol to fix when
building the MICROCHIP_PFSOC machine stand-alone:

  /usr/bin/ld: libcommon.fa.p/hw_char_mchp_pfsoc_mmuart.c.o: in function `mchp_pfsoc_mmuart_create':
  hw/char/mchp_pfsoc_mmuart.c:79: undefined reference to `serial_mm_init'

Fixes: a8fb0a500a6 ("hw/char: Add Microchip PolarFire SoC MMUART emulation")
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210515173716.358295-7-philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>