soc/intel: Replace SA_PCIEX_LENGTH Kconfig options

Use the existing `MMCONF_BUS_NUMBER` and `MMCONF_LENGTH` symbols.

Change-Id: I88dcc0d5845198f668c6604c45fd869617168231
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50113
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/security/intel/stm/StmPlatformResource.c

@@ -98,7 +98,7 @@ MSR_TABLE_ENTRY msr_table[] = {
 static void fixup_pciex_resource(void)
 {
 	// Find max bus number and PCIEX length
-	rsc_pcie_mmio.length = CONFIG_SA_PCIEX_LENGTH; // 0x10000000;// 256 MB
+	rsc_pcie_mmio.length = CONFIG_MMCONF_LENGTH; // 0x10000000;// 256 MB
 	rsc_pcie_mmio.base = CONFIG_MMCONF_BASE_ADDRESS;
 }