of: Add support for reading a u32 from a multi-value property.

This patch adds an of_property_read_u32_index() function to allow
reading a single indexed u32 value from a property containing multiple
u32 values.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robherring2@gmail.com>