qcom: spmi-wled: Limit FS current to 20 mA for PM855L v1.0

AS per the hardware recommendation, limit the full scale current
to 20 mA for PM855L v1.0. Since this needs to know the PMIC
revision, add support for reading it.

CRs-Fixed: 2197042
Change-Id: I0314b75291dfa8a70ce30cd36abfb78538dcefdd
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>