Change in coreboot[master]: cpu/x86/mtrr: Use a Kconfig for reserving MTRRs for OS

[Angel Pons (Code Review)]

Attention is currently required from: Nico Huber, Furquan Shaikh, Patrick Rudolph, Stefan Reinauer, Tim Wawrzynczak, Subrata Banik, Arthur Heymans, Aaron Durbin.
Angel Pons has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/52522 )

Change subject: cpu/x86/mtrr: Use a Kconfig for reserving MTRRs for OS
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Patch Set 6:

(1 comment)

File src/cpu/x86/mtrr/mtrr.c:

https://review.coreboot.org/c/coreboot/+/52522/comment/f5be6371_9427662d 
PS6, Line 711: wb_deftype_count > bios_mtrrs && uc_deftype_count > bios_mtrrs
> If we keep `bios_mtrrs` around, then we would need the check for CONFIG(RESERVE_MTRRS_FOR_OS) somewh […]

The main reason to do the reordering is to avoid printing "Taking a reserved OS MTRR." when we have run out of MTRRs.

If we decide to drop the `bios_mtrrs` variable, I would introduce an `os_mtrrs` constant for clarity:

if (var_state->mtrr_index >= total_mtrrs) {
    	printk(BIOS_ERR, "ERROR: Not enough MTRRs available! MTRR index is %d with %d MTRRs in total.\n",
    	       var_state->mtrr_index, total_mtrrs);
    	return;
    }
    const int os_mtrrs = CONFIG(RESERVE_MTRRS_FOR_OS) ? 2 : 0;
    if (var_state->mtrr_index >= total_mtrrs - os_mtrrs)
    	printk(BIOS_WARNING, "Taking a reserved OS MTRR.\n");

I would expect that gcc inlines the `os_mtrrs` constant and, if CONFIG_RESERVE_MTRRS_FOR_OS is not set, optimises away the second check.

Note: I have used `int` because it is what the current code uses. If the values are never negative, I would use `unsigned int` instead.