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/malta_defconfig b/configs/malta_defconfig
index 53762a9..ed41e29 100644
--- a/configs/malta_defconfig
+++ b/configs/malta_defconfig
@@ -44,3 +44,4 @@
 CONFIG_PCI_MSC01=y
 CONFIG_RTC_MC146818=y
 CONFIG_SYS_NS16550=y
+CONFIG_SYS_NS16550_PORT_MAPPED=y