arm: dts: k3-j7200-r5: Add fs_loader node

Add a generic fs_loader node to the K3 J7200 R5 common board dts
file and use it as the chosen firmware-loader so that it can be
used for loading various firmwares from a boot media/filesystem
in R5 SPL on K3 J7200 EVM.

Signed-off-by: Suman Anna <s-anna@ti.com>