drivers: ftm: fix compiler warning when clocks disabled

[manuargue]

Add missing guards to avoid compilation warnings when building with clock control driver disabled.

Prerequisites

• [x] I have checked latest main branch and the issue still exists.
• [x] I did not see it is stated as known-issue in release notes.
• [x] No similar GitHub issue is related to this change.
• [x] My code follows the commit guidelines of this project.
• [x] I have performed a self-review of my own code.
• [x] My changes generate no new warnings.
• [ ] I have added tests that prove my fix is effective or that my feature works.

Describe the pull request Add missing guards to avoid compilation warnings when building FTM driver with clock control driver disabled. Example of warning without this patch:

Building C object modules/hal_nxp...dir/mcux/mcux-sdk/drivers/ftm/fsl_ftm.c.obj
/home/user/zephyrproject/modules/hal/nxp/mcux/mcux-sdk/drivers/ftm/fsl_ftm.c:72:17: warning: 'FTM_GetInstance' defined but not used [-Wunused-function]
   72 | static uint32_t FTM_GetInstance(FTM_Type *base)
      |

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Tests

• Test configuration (please complete the following information):
  • Hardware setting:
  • Toolchain: Zephyr SDK
  • Test Tool preparation:
  • Any other dependencies:
• Test executed Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.
  • [x] Build Test
  • [x] Run Test