Change in coreboot[master]: arch/riscv: Document mprv_{read, write}_* functions

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Change subject: arch/riscv: Document mprv_{read,write}_* functions
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Patch Set 1: Verified+1

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https://qa.coreboot.org/job/coreboot-checkpatch/14999/ : SUCCESS

https://qa.coreboot.org/job/coreboot-gerrit/59729/ : SUCCESS