board: stemmy: Increase boot image to 64 MB

When using a recent kernel with a bunch of compiled-in
stuff the kernel image easily becomes bigger than 8 MB
yielding this error:

  Loading Kernel Image
  Image too large: increase CONFIG_SYS_BOOTM_LEN
  Must RESET board to recover

Fix this by bumping to SZ_64MB.

Cc: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stephan Gerhold <stephan@gerhold.net>