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/am43xx_hs_evm_qspi_defconfig b/configs/am43xx_hs_evm_qspi_defconfig
index 68a9651..08cac40 100644
--- a/configs/am43xx_hs_evm_qspi_defconfig
+++ b/configs/am43xx_hs_evm_qspi_defconfig
@@ -55,7 +55,7 @@
 CONFIG_SPI_FLASH_MACRONIX=y
 CONFIG_MII=y
 CONFIG_DRIVER_TI_CPSW=y
-CONFIG_SYS_NS16550=y
+CONFIG_SYS_NS16550_SERIAL=y
 CONFIG_SPI=y
 CONFIG_TI_QSPI=y
 CONFIG_USB=y