arch/x86/include/arch/smp/mpspec: improve mp_bustype enum definition

Since the raw values of the enum elements are used, explicitly assign
the value 0 to the first element to make it clearer that the absolute
values matter here.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I69f58cca7130ce5f0ebe4743754e4e31f55db289
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55615
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/arch/x86/include/arch/smp/mpspec.h

@@ -155,7 +155,7 @@ struct mpc_config_lintsrc {
 #define MAX_IRQ_SOURCES 128
 #define MAX_MP_BUSSES 32
 enum mp_bustype {
-	MP_BUS_ISA,
+	MP_BUS_ISA = 0,
 	MP_BUS_EISA,
 	MP_BUS_PCI,
 	MP_BUS_MCA