serial: 8250: simplify ralink/alchemy register remap selection

Some SoCs, including Ralink/Mediatek and Alchemy Au1xxx, have a
16550-like UART with a non-standard register layout.  These are
supported by a simple mapping table in 8250_port.c  Rather than
list every SoC type using this access mode in the ifdefs there,
allow selecting the SERIAL_8250_RT288X Kconfig option with any
system and default it to y for the known cases needing it.  The
help text is reworded accordingly.

This change simplifies adding support for other SoCs also using
the same UART.

The name of the option is a little misleading, but not knowing
the true origin of this UART, it is as good a choice as any.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>