ARM: hw_breakpoint: correct and simplify alignment fixup code

The current hw_breakpoint code tries to fix up the alignment of
breakpoints so that we can make use of sparse byte-address-select
bits in the control register and give the illusion that we can
set breakpoints on unaligned addresses.

Although this works on v6 cores, v7 forbids this behaviour, instead
requiring breakpoints to be set on aligned addresses and have contiguous
byte-address-select ranges depending on the instruction set in use.
For ARM the only supported size is 4 bytes, whilst Thumb-2 also permits
2 byte breakpoints (watchpoints can be of 1, 2, 4 or 8 bytes long).

This patch simplifies the alignment fixup code so that we require
addresses to be aligned to the size of the corresponding breakpoint.
This allows us to handle the common case of breaking on a half-word
aligned Thumb-2 instruction and also allows us to set byte watchpoints
on arbitrary addresses.

Signed-off-by: Will Deacon <will.deacon@arm.com>