[kmac] Add Endianness section in Programmer's guide
[kmac] Add Endianness section in Programmer's guide

The KMAC spec lacked of endian description. This commit is to add the
endian section to help the software engineer to understand KMAC's
internal operation and also the CFG.msg_endianness and
CFG.state_endianness behaviors.

Signed-off-by: Eunchan Kim <cb7bc7ceb22698f298c67faa994fd9ddf3cab298@opentitan.org>

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