arm/dts: msm8610 : Changes for wled homerow backlight.

The WLED homerow backlight name wled-homerow is replaced by
button-backlight. Also set the default current value written
to LED_MPP_SINK_CTRL register of MPP2 block of PMIC used in
8x10 QRD to 3 to set the default brightness of homerow backlight.

CRs-Fixed: 539440
Change-Id: I760b72a9d67d3b152e71143a4629be88ec95319a
Signed-off-by: Sarada Prasanna Garnayak <c_sgarna@codeaurora.org>