ARM: hw_breakpoint: Clear breakpoints before enabling monitor mode

The reset value of the BCR, BVR, WCR, and WVR registers are all
UNKNOWN on ARMv7. Unfortunately, reset_ctrl_regs() clears these
registers *after* enabling monitor mode, not before, and so some
implementations may experience UNPREDICTABLE behavior if the
reset values of these registers are non-zero. Clear the
breakpoints before enabling monitor mode so that we don't
experience boot hangs/loops due to breakpoints being enabled
out of reset.

Change-Id: I029fbe40725e803183544ddd6fe40285de4137e8
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>