dts: bindings: lsm303agr: add new bindings based on lis2dh

Add new bindings for already supported (by lis2dh.c driver) lsm303agr
accelerometer. Using st,lsm303agr-accel compatible will allow in
subsequent commit to apply different sensitivity scale.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>