ARM: cpuidle: Avoid orphan section warning

Since commit 83109d5d ("x86/build: Warn on orphan section placement"),
we get a warning for objects in orphan sections. The cpuidle implementation
for OMAP causes this when CONFIG_CPU_IDLE is disabled:

arm-linux-gnueabi-ld: warning: orphan section `__cpuidle_method_of_table' from `arch/arm/mach-omap2/pm33xx-core.o' being placed in section `__cpuidle_method_of_table'
arm-linux-gnueabi-ld: warning: orphan section `__cpuidle_method_of_table' from `arch/arm/mach-omap2/pm33xx-core.o' being placed in section `__cpuidle_method_of_table'
arm-linux-gnueabi-ld: warning: orphan section `__cpuidle_method_of_table' from `arch/arm/mach-omap2/pm33xx-core.o' being placed in section `__cpuidle_method_of_table'

Change the definition of CPUIDLE_METHOD_OF_DECLARE() to silently
drop the table and all code referenced from it when CONFIG_CPU_IDLE
is disabled.

Fixes: 06ee7a95 ("ARM: OMAP2+: pm33xx-core: Add cpuidle_ops for am335x/am437x")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201230155506.1085689-1-arnd@kernel.org