serial: pxa: use kconfig for serial configuration

Migrate the PXA serial driver to be configured via Kconfig.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 56c024f..620dd82 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -407,4 +407,10 @@
 	  for example APQ8016 and MSM8916.
 	  Single baudrate is supported in current implementation (115200).
 
+config PXA_SERIAL
+	bool "PXA serial port support"
+	help
+	  If you have a machine based on a Marvell XScale PXA2xx CPU you
+	  can enable its onboard serial ports by enabling this option.
+
 endmenu