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/rock960-rk3399_defconfig b/configs/rock960-rk3399_defconfig
index 6ed5ef8..f2c914f 100644
--- a/configs/rock960-rk3399_defconfig
+++ b/configs/rock960-rk3399_defconfig
@@ -65,6 +65,7 @@
 CONFIG_RNG_ROCKCHIP=y
 CONFIG_BAUDRATE=1500000
 CONFIG_DEBUG_UART_SHIFT=2
+CONFIG_SYS_NS16550_MEM32=y
 CONFIG_SYSRESET=y
 CONFIG_USB=y
 CONFIG_USB_XHCI_HCD=y