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/chromebook_samus_tpl_defconfig b/configs/chromebook_samus_tpl_defconfig
index e922264..7773573 100644
--- a/configs/chromebook_samus_tpl_defconfig
+++ b/configs/chromebook_samus_tpl_defconfig
@@ -91,6 +91,7 @@
 # CONFIG_SPL_PINCTRL is not set
 # CONFIG_TPL_PINCTRL is not set
 CONFIG_SYS_NS16550=y
+CONFIG_SYS_NS16550_PORT_MAPPED=y
 CONFIG_SOUND=y
 CONFIG_SOUND_I8254=y
 CONFIG_SOUND_RT5677=y