x86: Add a function to set variable MTRRs

x86: Add a function to set variable MTRRs

Normally U-Boot handles MTRRs through an add/commit process which
overwrites all MTRRs. But in very early boot it is not desirable to clear
the existing MTRRs since they may be in use and it can cause a hang.

Add a new mtrr_set_next_var() function which sets up the next available
MTRR to the required region.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: pass 'type' to set_var_mtrr() in mtrr_set_next_var()]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>