ARM: 7681/1: hw_breakpoint: use warn_once to avoid spam from reset_ctrl_regs()

CPU debug features like hardware break, watchpoints can be used only
when the debug mode is enabled and available. Unfortunately on OMAP4
based devices, after a CPU power cycle, the debug feature gets disabled
which leads to a flood of messages coming from reset_ctrl_regs() which
gets called on every CPU_PM_EXIT with CPUidle enabled.

So make use of warn_once() so that system is usable.

Thanks to Will for pointers and Lokesh for the analysis of the issue.

Tested-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>