soc/intel/xeon_sp/cpx: Disable isoch operation for performance

Isochronous operation negatively impacts memory performance, as
per Intel MLC (Memory Latency Checker) benchmark results. Thus,
disable isochronous operation, like analogous UEFI firmware does.
The MLC results after disabling isoch:

"--max_bandwidth"
ALL Reads :         106948.17
3:1 Reads-Writes :  101580.46
2:1 Reads-Writes :  100523.26
1:1 Reads-Writes :  99059.44
Stream-triad like : 97762.47

"--peak_injection_bandwidth"
ALL Reads :         105724.3
3:1 Reads-Writes :  100655.8
2:1 Reads-Writes :  99463
1:1 Reads-Writes :  98708
Stream-triad like : 91515

The MLC results before disabling isoch:

"--max_bandwidth"
ALL Reads :         88824.96
3:1 Reads-Writes :  94820.81
2:1 Reads-Writes :  94867.53
1:1 Reads-Writes :  92567.36
Stream-triad like : 91900.43

"--peak_injection_bandwidth"
ALL Reads :         88859.6
3:1 Reads-Writes :  94064
2:1 Reads-Writes :  94186.2
1:1 Reads-Writes :  92516.1
Stream-triad like : 85147.4

TEST=On OCP Delta Lake, verify that MLC benchmark results have
improved.

Change-Id: I08c22ee001b601e607452b3f23fad969ecb484b4
Signed-off-by: Johnny Lin <johnny_lin@wiwynn.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48738
Reviewed-by: Marc Jones <marc@marcjonesconsulting.com>
Reviewed-by: Jonathan Zhang <jonzhang@fb.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/xeon_sp/cpx/romstage.c

@@ -163,5 +163,8 @@ void platform_fsp_memory_init_params_cb(FSPM_UPD *mupd, uint32_t version)
 	m_cfg->VtdSupport = config->vtd_support;
 	m_cfg->X2apic = config->x2apic;
 
+	/* Disable ISOC */
+	m_cfg->isocEn = 0;
+
 	mainboard_memory_init_params(mupd);
 }