rs6000: Allow conversions of MMA pointer types [PR106017]

GCC incorrectly disables conversions between MMA pointer types, which
are allowed with clang.  The original intent was to disable conversions
between MMA types and other other types, but pointer conversions should
have been allowed.  The fix is to just remove the MMA pointer conversion
handling code altogether.

gcc/
	PR target/106017
	* config/rs6000/rs6000.cc (rs6000_invalid_conversion): Remove handling
	of MMA pointer conversions.

gcc/testsuite/
	PR target/106017
	* gcc.target/powerpc/pr106017.c: New test.