ARM: shmobile: kzm9g_defconfig: Update AK8975 config option

CONFIG_SENSORS_AK8975 was renamed to CONFIG_AK8975 in commit
2fc72cd8 ("iio:magnetometer:ak8975 move driver out of staging")
in v3.10.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>