UPSTREAM: power: supply: core: add capacity error margin property

Add a property for reporting the error margin expected
by fuel gauge chips.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
(cherry picked from commit bac705abcf345c28e419157cfcd1c44032cc9db2)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I95e67006d5e398b7cb902f9e96df245f0f85c6d2