boards: arm: 96b_neonkey: Fix I2C_3 for LP3943 LED driver

Following changes are necessary to use LP3943 LED driver present
in 96Boards Neonkey board

1. Enable I2C_3 interface
2. Reduce the speed of I2C_3 bus on 96Boards Neonkey. Due to some
design quirks, on board LP3943 LED driver connected to I2C_3
operates only at Standard speed of 100KHz.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>