sandbox: Rewrite i2c_pmic_emul.c to support PMIC with 3 bytes transmission

This change enables support for MC34708 PMIC in sandbox. Now we can
emulate the I2C transfers larger than 1 byte.

Notable changes for this driver:

- From now on the register number is not equal to index in the buffer,
  which emulates the PMIC registers

- The PMIC register's pool is now dynamically allocated up till
  64 regs * 3 bytes each = 192 B

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>