i2c_shell: Add write/read multiple-byte commands

Allow writing multiple registers (or a single 16-bit register) and
reading multiple registers.

   i2c read I2C_2 36 40
   00000000: 12 34 00 00 0F 3D 80 80 AF 9C 17 00 90 01 00 00

   i2c write I2C_2 36 40 12 34 56 78 90 12 34 56 78 90

   i2c read I2C_2 36 40
   00000000: 12 34 00 00 90 12 80 80 AF 9C 17 00 90 01 00 00

Signed-off-by: Simon Glass <sjg@chromium.org>