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: Will Deacon <will.deacon@arm.com>