brcmfmac: expose chip information through debugfs

Adding the debugfs file <debugfs_mnt>/brcmfmac/<devid>/chipinfo
which contains the chip number and revision.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>