boards: add axp2101-fuel-gauge to all boards with an AXP2101 PMIC

Multi-function PMIC AXP2101 now has a fuel gauge driver. Add the
corresponding node to all boards with such a PMIC (also checked their
datasheets and confirmed these actually have a battery correctly
connected to the PMIC).

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>