configs: am65x_evm: Define the maximum file size for DFU

In include/dfu.h, if CONFIG_SYS_DFU_MAX_FILE_SIZE is not defined then it is
defined as CONFIG_SYS_DFU_DATA_BUF_SIZE. This is 128 KiB for a53 core and
20 KiB for r5 core. If a larger file is transferred using dfu then it
fails.

CONFIG_SYS_DFU_DATA_BUF_SIZE can not be increased as there is not enough
heap memory to be allocated for the buffer in case of R5 spl.

Fix this by defining CONFIG_SYS_DFU_MAX_FILE_SIZE as the default
CONFIG_SYS_DFU_DATA_BUF_SIZE value.

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