linux-firmware: Update am65x/j721e sysfw to v2019.10a

Update the Makefile to automatically fetch and build the v2019.10a
version of the sysfw binaries for both AM65x and J721E SoCs.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Andreas Dannenberg <dannenberg@ti.com>
Acked-by: Suman Anna <s-anna@ti.com>