arm: am57xx: Allow bootm to load larger kernels

linux-mainline with multi_v7_defconfig + Android configs takes more
space than regular TI Android kernel and bootm will fail to load it.
Let's increase max kernel size up to 64 MiB to make it possible to run
such kernel.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>