bootcount: Fix misaligned cache operation

1947c2d2 introduces cache line flushes for the bootcounter, but if
the start address is not aligned then the flush causes warnings of
the form:

  CACHE: Misaligned operation at range [4030b7fc, 4030b83c]

Align both the start and end of the buffer (possibly crossing multiple
lines).

Fixes: 1947c2d2 ("bootcount: flush after storing the bootcounter")
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Acked-by: Stefano Babic <sbabic@denx.de>