reset: pistachio: Re-enable driver selection

After the retirement of MACH_PISTACHIO, the Pistachio Reset Driver is no
longer auto-enabled when building a kernel for Pistachio systems.
Worse, the driver cannot be enabled by the user at all (unless
compile-testing), as the config symbol is invisible.

Fix this partially by making the symbol visible again when compiling for
MIPS, and dropping the useless default.  The user still has to enable
the driver manually when building a kernel for Pistachio systems,
though.

Fixes: 104f942b ("MIPS: Retire MACH_PISTACHIO")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Link: https://lore.kernel.org/r/2c399e52540536df9c4006e46ef93fbccdde88db.1631610825.git.geert+renesas@glider.be


Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>