uart: ns16550: use io-mapped DT property for IO port access

The old CONFIG_UART_NS16550_ACCESS_IOPORT has been used to
indicate whether to access the NS16550 UART via IO port
before device tree is used to describe hardware. Now we have
device tree, and we can specify whether a particular UART
needs to be accessed via IO port using property io-mapped.
Therefore, CONFIG_UART_NS16550_ACCESS_IOPORT is no longer
needed (and thus also CONFIG_UART_NS16550_SIMULT_ACCESS).
Remove these two kconfigs and modify code to use device tree
to figure out how to access the UART hardware.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>