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/qemu-x86_64_defconfig b/configs/qemu-x86_64_defconfig
index 8433b57..0f4811e 100644
--- a/configs/qemu-x86_64_defconfig
+++ b/configs/qemu-x86_64_defconfig
@@ -70,6 +70,7 @@
 CONFIG_CPU=y
 CONFIG_NVME_PCI=y
 CONFIG_SPL_DM_RTC=y
+CONFIG_SYS_NS16550_PORT_MAPPED=y
 CONFIG_SPI=y
 CONFIG_USB_KEYBOARD=y
 CONFIG_FRAMEBUFFER_SET_VESA_MODE=y