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/configs/omap4_sdp4430_defconfig b/configs/omap4_sdp4430_defconfig
index 2315a84..e15c6d5 100644
--- a/configs/omap4_sdp4430_defconfig
+++ b/configs/omap4_sdp4430_defconfig
@@ -43,7 +43,7 @@
 CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_CONS_INDEX=3
-CONFIG_SYS_NS16550=y
+CONFIG_SYS_NS16550_SERIAL=y
 CONFIG_USB=y
 CONFIG_USB_OMAP3=y
 CONFIG_USB_GADGET=y