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/ls1021aqds_nor_SECURE_BOOT_defconfig b/configs/ls1021aqds_nor_SECURE_BOOT_defconfig
index 2f1b41f..40228ac 100644
--- a/configs/ls1021aqds_nor_SECURE_BOOT_defconfig
+++ b/configs/ls1021aqds_nor_SECURE_BOOT_defconfig
@@ -96,7 +96,7 @@
 CONFIG_PCIE_LAYERSCAPE_RC=y
 CONFIG_SYS_QE_FW_ADDR=0x60940000
 CONFIG_DM_SCSI=y
-CONFIG_SYS_NS16550=y
+CONFIG_SYS_NS16550_SERIAL=y
 CONFIG_USB=y
 CONFIG_USB_XHCI_HCD=y
 CONFIG_USB_XHCI_DWC3=y