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/cpu/x86/mp_init.c

@@ -218,7 +218,6 @@ static void setup_default_sipi_vector_params(struct sipi_params *sp)
 	sp->stack_top = ALIGN_DOWN((uintptr_t)&_estack, CONFIG_STACK_SIZE);
 }
 
-#define NUM_FIXED_MTRRS 11
 static const unsigned int fixed_mtrrs[NUM_FIXED_MTRRS] = {
 	MTRR_FIX_64K_00000, MTRR_FIX_16K_80000, MTRR_FIX_16K_A0000,
 	MTRR_FIX_4K_C0000, MTRR_FIX_4K_C8000, MTRR_FIX_4K_D0000,