include/cpu/x86/mtrr: define NUM_FIXED_MTRRS once in mtrr.h

Instead of defining NUM_FIXED_MTRRS in both cpu/x86/mp_init.h and
cpu/x86/mtrr/mtrr.c in two different ways that will evaluate to the same
value, define it once in include/cpu/x86/mtrr.h which is included in
both C files.

TEST=Timeless build for amd/mandolin results in identical firmware image

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I71cec61e22f5ce76baef21344c7427be29f193f8
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67774
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Martin Roth <martin.roth@amd.corp-partner.google.com>

src/include/cpu/x86/mtrr.h

@@ -44,6 +44,7 @@
 
 #define NUM_FIXED_RANGES		88
 #define RANGES_PER_FIXED_MTRR		8
+#define NUM_FIXED_MTRRS			(NUM_FIXED_RANGES / RANGES_PER_FIXED_MTRR)
 #define MTRR_FIX_64K_00000		0x250
 #define MTRR_FIX_16K_80000		0x258
 #define MTRR_FIX_16K_A0000		0x259