Fix KL46Z build

[multiplemonomials]

Summary of changes

I noticed that I made a mistake in #433 : I got confused about #ifdefs and thought that a given block of code was for KL46Z, when in fact it has been for KL43Z, which is a target that no longer exists in Mbed. This was then causing KL46Z to fail to compile, because it was compiling code for the wrong CPU!

As it turns out, the code in clk_freqs.h and clk_freqs.c is for both KL25Z and KL46Z!

Impact of changes

• Build fixed for KL46Z

Migration actions required

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

Able to compile all of Mbed for KL46Z now. Cannot test on hardware as I do not have one of these boards.

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