nvme: Add OAES bit-field definitions

Create definitions for the Optional Asynchronous Events Supported (OAES)
values. Also adds a helper macro for the common use case of "mask and
shift". E.g.

value = NVME_CTRLR_DATA_OAES_NS_ATTR_MASK << NVME_CTRLR_DATA_OAES_NS_ATTR_SHIFT;

becomes

value = NVMEB(NVME_CTRLR_DATA_OAES_NS_ATTR);

Reviewed by: mav, imp
Differential Revision: https://reviews.freebsd.org/D34300