arm: mach-k3: am642: Load SYSFW binary and config from boot media

Use the System Firmware (SYSFW) loader framework to load and start
the SYSFW as part of the AM642 early initialization sequence. Also
make use of existing logic to detect if ROM has already loaded sysfw
and avoided attempting to reload and instead just prepare to use already
running firmware.

While at it also initialize the MAIN_UART1 pinmux as it is used by SYSFW
to print diagnostic messages.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>