power: supply: axp288_fuel_gauge: Take the P-Unit semaphore only once during probe()

The I2C-bus to the XPower AXP288 is shared between the Linux kernel and
the SoCs P-Unit. The P-Unit has a semaphore which the kernel must "lock"
before it may use the bus. If not explicitly taken by the I2C-driver,
then this semaphore is automatically taken by the I2C-bus-driver for
each I2C-transfer and this is a quite expensive operation.

Explicitly take the semaphore in probe() around the register-accesses
done during probe, so that this only needs to be done once, rather then
once per register-access.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>