arm: dts: phycore-am62x: Package TIFS Stub

Add support for packaging the TIFS Stub as it's required for basic Low
Power Modes like Deep Sleep.
The reason it is packaged using binman and not inherently as part of the
DM firmware is because for HS devices, customer owns the customer key
and only customer has access to it.
DM is release by TI, Since TI doesn't have access to the customer key it
cannot have a component that is signed by customer key.
Hence, it's left as part of binman to be signed and packaged.

While at it, also make sure it's documented in phycore-am62x

Reviewed-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Wadim Egorov <w.egorov@phytec.de>
Signed-off-by: Dhruva Gole <d-gole@ti.com>