Change in coreboot[master]: arch/riscv: Drop mret workaround

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Change subject: arch/riscv: Drop mret workaround
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Patch Set 1: Verified+1

Build Successful

https://qa.coreboot.org/job/coreboot-gerrit/61337/ : SUCCESS

https://qa.coreboot.org/job/coreboot-checkpatch/16481/ : SUCCESS