module: move n1sdp_i2c module as cdns_i2c module

N1Sdp product specific n1sdp_i2c module is renamed
as cdns_i2c and moved to top level core module folder.
This allows platforms which have the Cadence I2C to make
use of the generic module and reduces code duplication.

References to n1sdp platform code has been removed from
the module.

Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>
Change-Id: Id25b31521e8c89b17b3b6d521dad710590076be4