ARM: ux500: don't select LEDS_GPIO for snowball

Using 'select' in Kconfig is hard, a platform cannot just
enable a driver without also making sure that its subsystem
is there. Also, there is no actual code dependency between
the platform and the gpio leds driver.

Without this patch, building without LEDS_CLASS esults in:

drivers/built-in.o: In function `create_gpio_led.part.2':
governor_userspace.c:(.devinit.text+0x5a58): undefined reference to `led_classdev_register'
drivers/built-in.o: In function `gpio_led_remove':
governor_userspace.c:(.devexit.text+0x6b8): undefined reference to `led_classdev_unregister'

This reverts 8733f53c "ARM: ux500: Kconfig: Compile in leds-gpio
support for Snowball" that introduced the regression and did not
provide a helpful explanation.

In order to leave the GPIO LED code still present in normal
builds, this also enables the symbol in u8500_defconfig, in addition
to the other LED drivers that are already selected there.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Lee Jones <lee.jones@linaro.org>