am65x_evm: Allow bootm to load larger kernels

Bootm will fail to load kernels over 8MB, this is not enough
for our 64bit kernel images. Increase this to 64MB.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>