soft_i2c: prevent compiler warnings if driver does not use CPU Pins.

This patch fixes the following warnings, when using
the soft_i2c driver using no CPU pins on MPC82xx or MPC8xx
systems:

soft_i2c.c: In function 'send_reset':
soft_i2c.c:93: warning: unused variable 'immr'
soft_i2c.c: In function 'send_start':
soft_i2c.c:124: warning: unused variable 'immr'
soft_i2c.c: In function 'send_stop':
soft_i2c.c:146: warning: unused variable 'immr'
soft_i2c.c: In function 'send_ack':
soft_i2c.c:171: warning: unused variable 'immr'
soft_i2c.c: In function 'write_byte':
soft_i2c.c:196: warning: unused variable 'immr'
soft_i2c.c: In function 'read_byte':
soft_i2c.c:244: warning: unused variable 'immr'

Signed-off-by: Heiko Schocher <hs@denx.de>