mtrr: Define a function for obtaining free var mtrr

Instead of hard-coding var mtrr numbers in code, use this function to
identify the first available variable mtrr. If no such mtrr is
available, the function will return -1.

Change-Id: I2a1e02cdb45c0ab7e30609641977471eaa2431fd
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/14115
Tested-by: build bot (Jenkins)
Reviewed-by: Andrey Petrov <andrey.petrov@intel.com>

src/include/cpu/x86/mtrr.h

@@ -81,6 +81,7 @@ void x86_mtrr_check(void);
 
 #if !defined(__ASSEMBLER__) && defined(__PRE_RAM__) && !defined(__ROMCC__)
 void set_var_mtrr(unsigned reg, unsigned base, unsigned size, unsigned type);
+int get_free_var_mtrr(void);
 #endif
 
 /* Align up to next power of 2, suitable for ROMCC and assembler too.