ARM: hw_breakpoint: make boot quieter without CPUID feature registers

Booting on a v6 core without the CPUID feature registers (e.g. 1136)
leads to a noisy dmesg complaining about their absence.

This patch changes the pr_warning into a pr_warn_once to keep the log
quieter.
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm/kernel/hw_breakpoint.c

@@ -136,8 +136,8 @@ static u8 get_debug_arch(void)
 
 	/* Do we implement the extended CPUID interface? */
 	if (((read_cpuid_id() >> 16) & 0xf) != 0xf) {
-		pr_warning("CPUID feature registers not supported. "
-			   "Assuming v6 debug is present.\n");
+		pr_warn_once("CPUID feature registers not supported. "
+			     "Assuming v6 debug is present.\n");
 		return ARM_DEBUG_ARCH_V6;
 	}