cmd: CMD_CPU depends on CPU

Compiling with CONFIG_CMD_CPU=y and CONFIG_CPU=n fails with:

aarch64-linux-gnu-ld.bfd: cmd/built-in.o: in function `print_cpu_list':
/home/user/u-boot/cmd/cpu.c:34: undefined reference to `cpu_get_desc'
aarch64-linux-gnu-ld.bfd:
/home/user/u-boot/cmd/cpu.c:39: undefined reference to `cpu_get_info'
Segmentation fault (core dumped)
make: *** [Makefile:1757: u-boot] Error 139
make: *** Deleting file 'u-boot'

See error report in https://stackoverflow.com/questions/64678347

.

Add the missing build dependency.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>