mvebu: axp: refactor board_sat_r_get() and caller

Instead of calling board_sat_r_get() only for those boards providing the
satr11 value via I2C, call it for all boards and return static values
for those not using I2C.

In addition to that, make this a weak function to allow for board code
to override it.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>