i2c: pxa: move private definitions to i2c-pxa.c

Move driver-private definitions out of the i2c-pxa.h platform data
header file into the driver itself. Nothing outside of the driver
makes use of these constants.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Wolfram Sang <wsa@kernel.org>