serial: sh-sci: Add support for dynamic instances

On DT platforms, the sh-sci driver requires the presence of "serialN"
aliases in DT, from which instance IDs are derived.  If a DT alias is
missing, the drivers fails to probe the corresponding serial port.

This becomes cumbersome when considering DT overlays, as currently
there is no upstream support for dynamically updating the /aliases node
in DT.  Furthermore, even in the presence of such support, hardcoded
instance IDs in independent overlays are prone to conflicts.

Hence add support for dynamic instance IDs, to be used in the absence of
a DT alias.  This makes serial ports behave similar to I2C and SPI
buses, which already support dynamic instances.

Ports in use are tracked using a simple bitmask of type unsigned long,
which is sufficient to handle all current hardware (max. 18 ports).
The maximum number of serial ports is still fixed, and configurable
through Kconfig.  Range validation is done through both Kconfig and a
compile-time check.

Due to the fixed maximum number of serial ports, dynamic and static
instances share the same ID space.  Static instances added later are
rejected when conflicting with dynamic instances registered earlier.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>