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/MPC8548CDS_legacy_defconfig b/configs/MPC8548CDS_legacy_defconfig
index 852ac9a..b01919e 100644
--- a/configs/MPC8548CDS_legacy_defconfig
+++ b/configs/MPC8548CDS_legacy_defconfig
@@ -84,4 +84,4 @@
 CONFIG_DM_PCI_COMPAT=y
 CONFIG_PCIE_FSL=y
 CONFIG_CONS_INDEX=2
-CONFIG_SYS_NS16550=y
+CONFIG_SYS_NS16550_SERIAL=y