mpc52xx: Get rid of board-specific #ifdef's in cpu/mpc5xxx/ide.c

Total5200 and digsy MTC use I2C port 2 pins as a ATA chip select.
To avoid adding board-specific ifdefs to cpu/mpc5xxx/ide.c new
define CONFIG_SYS_ATA_CS_ON_I2C2 was introduced. It is used by
Total5200 and will be used by digsy MTC and other boards with
ATA CS on I2C pins.

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>