pinctrl: zynq: Use define directive for PIN_CONFIG_IO_STANDARD

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

drivers/pinctrl/pinctrl-zynq.c:985:18: warning: implicit conversion from
enumeration type 'enum zynq_pin_config_param' to different enumeration
type 'enum pin_config_param' [-Wenum-conversion]
        {"io-standard", PIN_CONFIG_IOSTANDARD, zynq_iostd_lvcmos18},
        ~               ^~~~~~~~~~~~~~~~~~~~~
drivers/pinctrl/pinctrl-zynq.c:990:16: warning: implicit conversion from
enumeration type 'enum zynq_pin_config_param' to different enumeration
type 'enum pin_config_param' [-Wenum-conversion]
        = { PCONFDUMP(PIN_CONFIG_IOSTANDARD, "IO-standard", 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.

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>