Most AXP PMICs feature a "startup source" register, which keeps
information about how the PMIC started operation. Bit 0 in there means
it has been started by "plugging in the power cable".

Define a symbol in each PMIC's header file to be able to use that
register and bit later on.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>