pinctrl: lpc18xx: Use define directive for PIN_CONFIG_GPIO_PIN_INT

Clang warns when one enumerated type is implicitly converted to another:

drivers/pinctrl/pinctrl-lpc18xx.c:643:29: warning: implicit conversion
from enumeration type 'enum lpc18xx_pin_config_param' to different
enumeration type 'enum pin_config_param' [-Wenum-conversion]
        {"nxp,gpio-pin-interrupt", PIN_CONFIG_GPIO_PIN_INT, 0},
        ~                          ^~~~~~~~~~~~~~~~~~~~~~~
drivers/pinctrl/pinctrl-lpc18xx.c:648:12: warning: implicit conversion
from enumeration type 'enum lpc18xx_pin_config_param' to different
enumeration type 'enum pin_config_param' [-Wenum-conversion]
        PCONFDUMP(PIN_CONFIG_GPIO_PIN_INT, "gpio pin int", NULL, true),
        ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/pinctrl/pinconf-generic.h:163:11: note: expanded from
macro 'PCONFDUMP'
        .param = a, .display = b, .format = c, .has_arg = d     \
                 ^
2 warnings generated.

It is expected that pinctrl drivers can extend pin_config_param because
of the gap between PIN_CONFIG_END and PIN_CONFIG_MAX so this conversion
isn't an issue. Most drivers that take advantage of this define the
PIN_CONFIG variables as constants, rather than enumerated values. Do the
same thing here so that Clang no longer warns.

Link: https://github.com/ClangBuiltLinux/linux/issues/140
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>