arm: K3: sysfw-loader: Allow loading SYSFW via Y-Modem

In order to allow booting TI K3 family SoCs via Y-Modem add support for
loading System Firmware by tapping into the associated SPL core loader
function.

In this context also make sure a console is available and if not go
ahead and activate the early console feature which allows bringing up
an alternate full console before the main console is activated. Such
an alternate console is typically setup in a way that the associated
UART can be fully initialized prior to SYSFW services being available.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>