arm: K3: common: Allow for early console functionality

Implement an early console functionality in SPL that can be used before
the main console is being brought up. This helps in situations where the
main console is dependent on System Firmware (SYSFW) being up and running,
which is usually not the case during the very early stages of boot. Using
this early console functionality will allow for an alternate serial port
to be used to support things like UART-based boot and early diagnostic
messages until the main console is ready to get activated.

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