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/d2net_v2_defconfig b/configs/d2net_v2_defconfig
index 5eea1ec..71c57ed 100644
--- a/configs/d2net_v2_defconfig
+++ b/configs/d2net_v2_defconfig
@@ -66,7 +66,8 @@
 CONFIG_SPI_FLASH_MACRONIX=y
 CONFIG_MVGBE=y
 CONFIG_MII=y
-CONFIG_SYS_NS16550=y
+CONFIG_SYS_NS16550_SERIAL=y
+CONFIG_SYS_NS16550_REG_SIZE=-4
 CONFIG_SPI=y
 CONFIG_DM_SPI=y
 CONFIG_KIRKWOOD_SPI=y