mb/emulation/qemu: Configure TSEG size

Configure TSEG size by reading CONFIG_SMM_TSEG_SIZE in romstage.
The remaining Qemu code can already handle the bigger TSEG region.

TEST: Increased TSEG to 8MiB.

Change-Id: I1ae5ac93ecca83ae9c319c666aac844bbd5b259f
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83114
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/cpu/qemu-x86/Kconfig

@@ -31,6 +31,11 @@ config CPU_QEMU_X86_TSEG_SMM
 
 endchoice
 
+config SMM_TSEG_SIZE
+	hex
+	depends on SMM_TSEG
+	default 0x100000
+
 config MAX_CPUS
 	int
 	default 32 if SMM_TSEG