i2c: i2c-mpc: add CONFIG_PM_SLEEP to suspend/resume functions

Add CONFIG_PM_SLEEP to suspend/resume functions to fix the following
build warning when CONFIG_PM_SLEEP is not selected. This is because
sleep PM callbacks defined by SIMPLE_DEV_PM_OPS are only used when
the CONFIG_PM_SLEEP is enabled.

drivers/i2c/busses/i2c-mpc.c:724:12: warning: 'mpc_i2c_suspend' defined but not used [-Wunused-function]
drivers/i2c/busses/i2c-mpc.c:734:12: warning: 'mpc_i2c_resume' defined but not used [-Wunused-function]

Also, this patch makes mpc_i2c_pm_ops static, because mpc_i2c_pm_ops
is not exported.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>