blackfin: Move blackfin serial driver out of blackfin arch folder.

- Move blackfin serial driver to the generic driver folder.
- Move blackfin serial headers to blackfin arch head folder.
- Update the include path to blackfin serial header in start up code.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index fbc4e97..442b7ea 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -52,6 +52,7 @@
 COBJS-$(CONFIG_SANDBOX_SERIAL) += sandbox.o
 COBJS-$(CONFIG_SCIF_CONSOLE) += serial_sh.o
 COBJS-$(CONFIG_ZYNQ_SERIAL) += serial_zynq.o
+COBJS-$(CONFIG_BFIN_SERIAL) += serial_bfin.o
 
 ifndef CONFIG_SPL_BUILD
 COBJS-$(CONFIG_USB_TTY) += usbtty.o