ena: Add support for device request reset message over AENQ

This commit adds a handler for the new aenq message
ENA_ADMIN_DEVICE_REQUEST_RESET,
which in turn causes the driver to trigger reset of a new type:
ENA_REGS_RESET_DEVICE_REQUEST. Also adds counting of such occurrences in
a new statistic for it.

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.