Change in coreboot[master]: soc/mediatek: Add timestamp to measure PERST# time

[Yu-Ping Wu (Code Review)]

Attention is currently required from: Hung-Te Lin, Rex-BC Chen, Paul Menzel, Angel Pons, Jianjun Wang.
Yu-Ping Wu has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/62933 )

Change subject: soc/mediatek: Add timestamp to measure PERST# time
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Patch Set 3:

(1 comment)

File src/soc/mediatek/common/pcie.c:

https://review.coreboot.org/c/coreboot/+/62933/comment/cb0764c3_2f6297ed 
PS3, Line 238: 	perst_time_us = mtk_pcie_perst_elapsed_time();
             : 	printk(BIOS_DEBUG, "%s: %lld us elapsed since assert PERST#\n",
             : 	       __func__, perst_time_us);
             : 
             : 	/*
             : 	 * Described in PCIe CEM specification sections 2.2
             : 	 * (PERST# Signal) and 2.2.1 (Initial Power-Up (G3 to S0)).
             : 	 * The deassertion of PERST# should be delayed 100ms (TPVPERL)
             : 	 * for the power and clock to become stable.
             : 	 */
             : 	if (perst_time_us < 100000)
             : 		printk(BIOS_WARNING, "%s: PERST# assert time %lld us may not enough for link up\n",
             : 		       __func__, perst_time_us);
             :
> I think we should do it like: […]

I wonder if we can store the stopwatch (3x size of mono_time) in SRAM. Then we can use stopwatch_wait_until_expired() here.