x86/sev: Register tpm-svsm platform device

SNP platform can provide a vTPM device emulated by SVSM.

The "tpm-svsm" device can be handled by the platform driver registered by the
x86/sev core code.

Register the platform device only when SVSM is available and it supports vTPM
commands as checked by snp_svsm_vtpm_probe().

  [ bp: Massage commit message. ]

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20250410135118.133240-5-sgarzare@redhat.com

Author: stefano-garzarella

arch/x86/coco/sev/core.c

@@ -2688,6 +2688,11 @@ static struct platform_device sev_guest_device = {
 	.id		= -1,
 };
 
+static struct platform_device tpm_svsm_device = {
+	.name		= "tpm-svsm",
+	.id		= -1,
+};
+
 static int __init snp_init_platform_device(void)
 {
 	if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
@@ -2696,7 +2701,11 @@ static int __init snp_init_platform_device(void)
 	if (platform_device_register(&sev_guest_device))
 		return -ENODEV;
 
-	pr_info("SNP guest platform device initialized.\n");
+	if (snp_svsm_vtpm_probe() &&
+	    platform_device_register(&tpm_svsm_device))
+		return -ENODEV;
+
+	pr_info("SNP guest platform devices initialized.\n");
 	return 0;
 }
 device_initcall(snp_init_platform_device);