configs: j72*_evm: Define the buffer sizes for dfu

On J721e R5 SPL, dfu buffer for loading sysfw.itb image gets allocated
before DRAM gets initialized. So, the buffer gets allocated in MCU L3
RAM. The current buffer size to be allocated is 256KB  and the available
total heap memory is 0x70000 (448KB). This leads to NOMEM errors during
allocation.

In other cases when constraints such as above are not present fix the size
of buffers to the sector size in OSPI for proper functioning.

Also, if CONFIG_SYS_DFU_DATA_BUF_SIZE is defined and
CONFIG_SYS_DFU_MAX_FILE_SIZE is not defined then the max file size for dfu
transfer is defined as CONFIG_SYS_DFU_DATA_BUF_SIZE.

Fix these by setting appropriate buffer sizes in their respective defconfig
files and defining the max file size as 8 MB which is the default dfu
buffer size.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>