Change in coreboot[master]: soc/amd/stoneyridge: Wait for UART to be ready

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Change subject: soc/amd/stoneyridge: Wait for UART to be ready
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Patch Set 3: Verified+1

Build Successful

https://qa.coreboot.org/job/coreboot-gerrit/61340/ : SUCCESS

https://qa.coreboot.org/job/coreboot-checkpatch/16484/ : SUCCESS