leds: lp8860: Remove unused of_gpio,h

of_gpio.h provides a single function, which is not used in this driver.
Remove unused header.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230215170403.84449-1-andriy.shevchenko@linux.intel.com