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>
diff --git a/arch/arm/mach-k3/am654_init.c b/arch/arm/mach-k3/am654_init.c
index 12d7463..2336da4 100644
--- a/arch/arm/mach-k3/am654_init.c
+++ b/arch/arm/mach-k3/am654_init.c
@@ -245,8 +245,8 @@
 	/* Output System Firmware version info */
 	k3_sysfw_print_ver();
 
-	if (IS_ENABLED(CONFIG_K3_BOARD_DETECT))
-		do_board_detect();
+	/* Perform board detection */
+	do_board_detect();
 
 #if defined(CONFIG_CPU_V7R) && defined(CONFIG_K3_AVS0)
 	ret = uclass_get_device_by_driver(UCLASS_MISC, DM_DRIVER_GET(k3_avs),