serial: Add missing TPL_SYS_NS16550_SERIAL symbol

On PowerPC platforms with TPL enabled and SPL_SYS_NS16550_SERIAL
enabled, today this builds under TPL as well due to how $(XPL_) is
defined. Add the TPL_SYS_NS16550_SERIAL itself for consistency and
clarity.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index c4f4a8d..8413052 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -789,6 +789,12 @@
 	default y if SYS_NS16550_SERIAL || ARCH_SUNXI || ARCH_OMAP2PLUS
 	select SYS_NS16550
 
+config TPL_SYS_NS16550_SERIAL
+	bool "NS16550 UART or compatible legacy driver in TPL"
+	depends on TPL && !TPL_DM_SERIAL
+	default y if SPL_SYS_NS16550_SERIAL
+	select SYS_NS16550
+
 config SYS_NS16550
 	bool "NS16550 UART or compatible"
 	help