bng_en: Add backing store support

bng_en: Add backing store support

Backing store or context memory on the host helps the
device to manage rings, stats and other resources.
Context memory is allocated with the help of ring
alloc/free functions.

Signed-off-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Bhargava Chenna Marreddy <bhargava.marreddy@broadcom.com>
Reviewed-by: Rajashekar Hudumula <rajashekar.hudumula@broadcom.com>
Link: https://patch.msgid.link/20250701143511.280702-7-vikas.gupta@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>