doc: announce end of support for some Broadcom devices
doc: announce end of support for some Broadcom devices

Devices belonging to BCM573xx and BCM5740x family will not be supported
from the 21.02 release.

Signed-off-by: Ajit Khaparde <01965fbd61169dc18f3fb5e13de7f67d298f8924@broadcom.com>
Reviewed-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>
Acked-by: Kevin Traynor <1f46c93815774a2e602547cf1e37c682d62b566d@redhat.com>
Acked-by: Jerin Jacob <352c4d4f9291b869992ddb67daa45ddc149fed30@marvell.com>
Acked-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk