arm: mach-k3: Add weak do_board_detect() to common file

arm: mach-k3: Add weak do_board_detect() to common file

This matches how it was done for pre-K3 TI platforms and it allows
us to move the forward declaration out of sys_proto.h.

It also removes the need for K3_BOARD_DETECT as one is free to simply
override the weak function in their board files as needed.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>