input: sensors: make MPU6050 "i2c-supply" property optional

Some devices do not have separate regulator for I2C bus pull-up,
the property "vi2c-supply" should be optional property. This change
updates MPU6050 driver to continue without this property, binding
document is also updated accordingly.

Change-Id: I77210eadb858392efdc13911ae1a0a03b346523c
Signed-off-by: Bingzhe Cai <bingzhec@codeaurora.org>