bnxt_en: Add Level 2 context memory paging support.

Add the new functions bnxt_alloc_ctx_pg_tbls()/bnxt_free_ctx_pg_tbls()
to allocate and free pages for context memory.  The new functions
will handle the different levels of paging support and allocate/free
the pages accordingly using the existing functions.

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