Convert CONFIG_SYS_NS16550_MEM32 et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_NS16550_MIN_FUNCTIONS
   CONFIG_SYS_NS16550_MEM32
   CONFIG_SYS_NS16550_PORT_MAPPED
   CONFIG_SYS_NS16550_REG_SIZE
   CONFIG_SYS_NS16550_SERIAL

To do this we also introduce CONFIG_SPL_SYS_NS16550_SERIAL so that
platforms can enable the legacy driver here for SPL.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index eb7b8f2..45cf94c 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -25,7 +25,7 @@
 obj-$(CONFIG_PL01X_SERIAL) += serial_pl01x.o
 else
 obj-$(CONFIG_PL011_SERIAL) += serial_pl01x.o
-obj-$(CONFIG_SYS_NS16550_SERIAL) += serial_ns16550.o
+obj-$(CONFIG_$(SPL_)SYS_NS16550_SERIAL) += serial_ns16550.o
 endif
 
 obj-$(CONFIG_ALTERA_UART) += altera_uart.o