[kmac/doc] Clarify purpose of entropy mode configuration values
[kmac/doc] Clarify purpose of entropy mode configuration values

The idle_mode should really only be used for the very initial ROM_CTRL
phase. After that software should swtich to either sw_mode or edn_mode
if the entropy complex is available and importantly also set
entropy_ready.

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

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