arm64: a37xx: pinctrl: Fix definitions for MPP pins 20-22

All 3 MPP pins (20, 21 and 22) can be configured individually and also can
be configured to GPIO functions. Fix definitions for these MPP pins in
existing pin groups. After this change GPIO function can be enabled just
for one of these 3 pins.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>