dm: i2c: Add compatibility functions for dm_i2c_reg_read/write()

Add the legacy i2c_reg_read/write() functions to the compatibility layer
so that they can be used when CONFIG_DM_I2C_COMPAT is defined.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>