* remove orphaned Kconfig symbol now that CONFIG_IIO is no longer
selected after commit ef8b935c95 ("mediatek: clean up mt7622 kernel config")
* select UBI fast-map feature to decrease boot time and keep the
number of spare blocks required in sync with U-Boot's expectations
(we got fast-map enabled in U-Boot)
Signed-off-by: Daniel Golle <daniel@makrotopia.org>