dt: add helper function to read u8 & u16 variables & arrays

This adds following helper routines:
- of_property_read_u8_array()
- of_property_read_u16_array()
- of_property_read_u8()
- of_property_read_u16()

This expects arrays from DT to be passed as:
- u8 array:
	property = /bits/ 8 <0x50 0x60 0x70>;
- u16 array:
	property = /bits/ 16 <0x5000 0x6000 0x7000>;

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>