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>