x86: early_serial: extend to support MMIO UART

This expands the early_serial to support MMIO UART, in addition to
port I/O, by duplicating part of the hardware initialization from
the NS16550 UART driver. This allows enabling of early console on
hardware with MMIO-based UARTs.

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