gpu: nvgpu: fix MISRA Rule 8.6 violations

ifdef function prototypes with CONFIG_* defines. This fixes MISRA rule
8.6 violations which complain about undefined functions.
Also moved nvgpu_channel_get_from_file prototype to ioctl_channel.h &
nvgpu_probe to driver_common.h as those are linux specific. Define
nvgpu_init_soc_vars in posix/soc.c as it is implemented in QNX.

JIRA NVGPU-3873

Change-Id: I5d2b238e1b5d1318867cd2416ac5f03cc6ab7c6a
Signed-off-by: Sagar Kamble <skamble@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/2196794
Reviewed-by: svc-mobile-coverity <svc-mobile-coverity@nvidia.com>
Reviewed-by: Deepak Nibade <dnibade@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Alex Waterman <alexw@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/os/posix/soc.c

@@ -66,3 +66,8 @@ u64 nvgpu_get_cycles(void)
 {
 	return (u64)nvgpu_current_time_us();
 }
+
+int nvgpu_init_soc_vars(struct gk20a *g)
+{
+	return 0;
+}