drivers/i2c/i2c_dw: really, really make 64-bit clean

The base_address in the device configuration is used as a handle
to access an entity in memory. In C, we call that a 'pointer'.
Also in C, (versus, say, PL/M) we name these pointer things after
what they point not ('regs') not what they are ('base address').

Thus, we change the member to a pointer type and change its name.
This makes it compile cleanly regardless of machine pointer size,
while also cutting down on a bunch of casting noise.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>