gpio-lpc32xx: Add GPI_28

This patch adds the missing gpi28 to the list of GPIOs in the GPI P3 "chip".

NOTE: This patch depends on incrementing LPC32XX_GPI_P3_MAX.  When applied
without the respective mach-lpc32xx patch (merged via arm-soc.git), gcc will
give a warning about "excess elements in array initializer" but this doesn't
harm.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>