scpi: hide get_scpi_ops in module from built-in code

The scpi_clock driver can be built-in when CONFIG_COMPILE_TEST
is set even when ARM_SCPI_PROTOCOL is a loadable module, and
that results in a link error:

drivers/built-in.o: In function `scpi_clocks_probe':
(.text+0x14453c): undefined reference to `get_scpi_ops'

Using #if IS_REACHABLE() around the get_scpi_ops() declaration
makes it build successfully in this case for compile-testing,
but the effect is the same as when ARM_SCPI_PROTOCOL is
disabled, as the code will not be used.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Punit Agrawal <punit.agrawal@arm.com>