boston: Bump CONFIG_SYS_BOOTM_LEN to 64MiB

The default value of CONFIG_SYS_BOOTM_LEN is too small for typical
boston Linux kernels. Increase the limit to 64MB, which covers current
kernels with plenty of breathing room.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>