configs: am335x_evm: Increase SPL_SYS_MALLOC_F_LEN to accomodate gpio_devices

With gpio devices getting created in SPL, the size of the heap is
no longer sufficient. Therefore, increase SPL_SYS_MALLOC_F_LEN
to 0x1000.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>