libpayload: add x86 ROM variable MTRR support

On x86, coreboot may allocate a variable range MTRR for enabling caching
of the system ROM. Add the ability to parse this structure and add the
result to the sysinfo structure.

An example usage implementation would be to obtain the variable MTRR
index that covers the ROM from the sysinfo structure. Then one would
disable caching and change the MTRR type from uncacheable to
write-protect and enable caching. The opposite sequence is required
to tearn down the caching.

Change-Id: I3bfe2028d8574d3adb1d85292abf8f1372cf97fa
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2920
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/include/sysinfo.h

@@ -99,6 +99,11 @@ struct sysinfo_t {
 	void	*vdat_addr;
 	u32	vdat_size;
 #endif
+
+#ifdef CONFIG_ARCH_X86
+	int x86_rom_var_mtrr_index;
+#endif
+
 	void	*tstamp_table;
 	void	*cbmem_cons;
 	void	*mrc_cache;