[Committed] Move new test case to gcc.target/avr/mmcu/pr54816.c

AVR test cases that specify a specific -mmcu option need to be placed
in the gcc.target/avr/mmcu subdirectory.  Moved thusly.

2023-04-22  Roger Sayle  <roger@nextmovesoftware.com>

gcc/testsuite/ChangeLog
	PR target/54816
	* gcc.target/avr/pr54816.c: Move to...
	* gcc.target/avr/mmcu/pr54816.c: ... here.

diff --git a/gcc/testsuite/gcc.target/avr/pr54816.c b/gcc/testsuite/gcc.target/avr/mmcu/pr54816.c
similarity index 100%
rename from gcc/testsuite/gcc.target/avr/pr54816.c
rename to gcc/testsuite/gcc.target/avr/mmcu/pr54816.c