leds: Add SN3218 and SN3216 support to the IS31FL32XX driver

Si-En Technology was acquired by ISSI in 2011, and it appears that
the IS31FL3218/IS31FL3216 are just rebranded SN3218/SN3216 devices.

Add the "si-en,sn3218" and "si-en,sn3216" compatible strings into the
IS31FL32XX driver as aliases for the issi equivalents, and update
binding documentation.

Datasheets:
    IS31FL3218: http://www.issi.com/WW/pdf/31FL3218.pdf
    SN3218:     http://www.si-en.com/uploadpdf/s2011517171720.pdf

    IS31FL3216: http://www.issi.com/WW/pdf/31FL3216.pdf
    SN3216:     http://www.si-en.com/uploadpdf/SN3216201152410148.pdf



Signed-off-by: David Rivshin <drivshin@allworx.com>
Acked-by: Rob Herring <robh@kernel.org>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>