spl: Commit MTRRs only in board_init_f_r()

We don't need to commit the SPI-flash MTRR change immediately, since it is
now done in the board_init_f_r(). Also this causes chromebook_link64 to
hang, presumably since we are still running from CAR (Cache-as-RAM) in
SPL. Coral handles this OK, perhaps since it is running from a different
memory area, but it has no effect on Coral anyway.

Drop the extra mtrr_commit() in the SPL implementation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>