soc/amd/stoneyridge: Align get_cpu_count to other targets

The CPUID function to get the number of cores on a package is common
across multiple generations of AMD cpus.

Change-Id: I28bff875ea2df7837e4495787cf8a4c2d522d43d
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64869
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>

src/soc/amd/stoneyridge/cpu.c

@@ -43,7 +43,7 @@ static void pre_mp_init(void)
 
 static int get_cpu_count(void)
 {
-	return (pci_read_config16(SOC_HT_DEV, D18F0_CPU_CNT) & CPU_CNT_MASK) + 1;
+	return 1 + (cpuid_ecx(0x80000008) & 0xff);
 }
 
 static const struct mp_ops mp_ops = {