MIPS: cm: Fix warning if MIPS_CM is disabled

Commit e27fbe16 ("MIPS: cm: Detect CM quirks from device tree")
introduced

arch/mips/include/asm/mips-cm.h:119:13: error: ‘mips_cm_update_property’
	defined but not used [-Werror=unused-function]

Fix this by making empty function implementation inline

Fixes: e27fbe16 ("MIPS: cm: Detect CM quirks from device tree")
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>