bnxt_en: add support for HWRM request slices

Slices are a mechanism for suballocating DMA mapped regions from the
request buffer. Such regions can be used for indirect command data
instead of creating new mappings with dma_alloc_coherent().

The advantage of using a slice is that the lifetime of the slice is
bound to the request and will be automatically unmapped when the
request is consumed.

A single external region is also supported. This allows for regions
that will not fit inside the spare request buffer space such that
the same API can be used consistently even for larger mappings.

Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>