[otbn] Fix operation description for BN.RSHI
[otbn] Fix operation description for BN.RSHI

* Indicate in Python syntax the concatenation of two registers.
* Use consistent names for decoded values.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan