gpio: lpc18xx: convert GPIO_LPC18XX from bool to tristate

The Kconfig currently controlling compilation of this code is:

 config GPIO_LPC18XX
       bool "NXP LPC18XX/43XX GPIO support"

...meaning that it currently is not being built as a module by anyone.

When targeting orphaned modular code in non-modular drivers, this
came up.  Joachim indicated that the driver was actually meant to
be tristate but ended up bool by accident.  So here we make it
tristate instead of removing the modular code that was essentially
orphaned.

Cc: Joachim Eastwood <manabian@gmail.com>
Acked-by: Joachim Eastwood <manabian@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>