[OTBN] fix inconsitency in spec
[OTBN] fix inconsitency in spec

This is related to lowRISC#2570

The CMP/CMPB instructions are supposed to share the same logic
as the SUB/SUBB instructions, with the only difference that
no result is written in the case of CMP/CMPB.
This adds the possible immediate shift of rs2 to the CMP and CMPB
instructions.

Signed-off-by: Felix Miller <43d2a12732a784b305469416c0ba5ce1795e752a@gi-de.com>

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