i2c_shell: Add write_byte/read_byte commands

Add simple commands to read or write a single byte from a device
register.

   i2c write_byte I2C_2 36 b0 12
   i2c read_byte I2C_2 36 0
   Output: 0x82

I modified Anas' version to put args in variables first so that the code
is self-documenting.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
Signed-off-by: Simon Glass <sjg@chromium.org>