board: disco_l475_iot1: fix POLL dependency

Adding 'config POLL' in Kconfig.defconfig creates a dependency
between CONFIG_POLL and board BOARD_DISCO_L475_IOT1 & SPI.
Remove this definition from the file and set CONFIG_POLL
in board _defconfig file.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>