[ot] hw/opentitan: ot_keymgr: Implement basic reg read/writes

Implements the logic for keymgr register reads and writes, mostly
limited to write masking, RW0C/RW1C/RO/WO behaviour, REGWENs and
shadowed registers. A small bit of additional logic is added to hook
up the interrupt/alert state/test registers to propagate IRQs and
alert signals.

Some logic is also implemented to load the salt and sealing keys into
separate byte arrays, since these will be needed for later parts of the
implementation. The RC (read-clear) software share outputs are left
completely unimplemented for now as these will be added later along with
their implementation.

Signed-off-by: Alex Jones <[email protected]>

Author: AlexJones0