marvell: drivers: use anonymous union in I2C driver

marvell: drivers: use anonymous union in I2C driver

The I2C controller found in Marvell A8K SoCs (and some older SoCs) mux
status and baudrate registers into the same address, however, it's a
vendor customization, and the original IP core by Mentor Graphics uses
two different addresses for the two registers.

Use anonymous union in the driver, in order to ease code sharing for
other SoC vendors that use this IP core (Allwinner SoCs that are newly
introduced to mainline ATF use this core).

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>