include/cpu/amd/msr: introduce and use PSTATE_MSR_COUNT

Add and use a define for the total number of P-state MSRs to avoid magic
constants in the code.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I37a89faa0f216790b3404fc03edc62408684cc24
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76546
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

src/include/cpu/amd/msr.h

@@ -41,6 +41,7 @@
 #define PS_STS_REG			0xC0010063
 #define PSTATE_0_MSR			0xC0010064
 #define PSTATE_MSR(pstate)		(PSTATE_0_MSR + (pstate))
+#define PSTATE_MSR_COUNT		8
 
 #define MSR_PATCH_LOADER		0xC0010020