x86: Return mtrr_add_request() to its old purpose

This function used to be for adding a list of requests to be actioned on
relocation. Revert it back to this purpose, to avoid problems with boards
which need control of their MTRRs (i.e. those which don't use FSP).

The mtrr_set_next_var() function is available when the next free
variable-MTRR must be set, so this can be used instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Fixes: 3bcd6cf8 ("x86: mtrr: Skip MSRs that were already programmed..")
Fixes: 596bd058 ("x86: mtrr: Do not clear the unused ones..")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>