hw/mips: Explicit the semi-hosting feature is always required

Disabling the semi-hosting feature leads to build failure:

    LINK    mips-softmmu/qemu-system-mips
  /usr/bin/ld: target/mips/mips-semi.o: in function `helper_do_semihosting':
  target/mips/mips-semi.c:335: undefined reference to `qemu_semihosting_log_out'
  /usr/bin/ld: target/mips/mips-semi.c:338: undefined reference to `qemu_semihosting_log_out'
  collect2: error: ld returned 1 exit status

Add a comment to avoid this feature to be disabled.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-Id: <20190701112612.14758-2-philmd@redhat.com>

default-configs/mips-softmmu-common.mak

 # Common mips*-softmmu CONFIG defines
 
+# CONFIG_SEMIHOSTING is always required on this architecture
+CONFIG_SEMIHOSTING=y
+
 CONFIG_ISA_BUS=y
 CONFIG_PCI=y
 CONFIG_PCI_DEVICES=y
@@ -35,7 +38,6 @@ CONFIG_MIPS_CPS=y
 CONFIG_MIPS_ITU=y
 CONFIG_R4K=y
 CONFIG_MALTA=y
-CONFIG_SEMIHOSTING=y
 CONFIG_PCNET_PCI=y
 CONFIG_MIPSSIM=y
 CONFIG_ACPI_SMBUS=y