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/socfpga_n5x_defconfig b/configs/socfpga_n5x_defconfig
index fa19f55..1056932 100644
--- a/configs/socfpga_n5x_defconfig
+++ b/configs/socfpga_n5x_defconfig
@@ -72,6 +72,7 @@
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_MII=y
 CONFIG_DM_RESET=y
+CONFIG_SYS_NS16550_MEM32=y
 CONFIG_SPI=y
 CONFIG_CADENCE_QSPI=y
 CONFIG_DESIGNWARE_SPI=y