configs: am62x_evm_*: Set DFU buffer size to 256k

configs: am62x_evm_*: Set DFU buffer size to 256k

OSPI flash writing was broken on am62x SKs as the OSPI flash device has a
256k sector size but was set to 20k. This breaks OSPI flash writes because
the current implementation of the SF DFU backend requires that the DFU data
buffer size be the same as the sector erase size, and the USB DFU config
fragment (for both A53 and R5) reduced the size of the DFU data buffer,
presumably so that it can fit into SPL's much smaller heap.

Change the SYS_DFU_DATA_BUF_SIZE to the erase sector size, which still fits
within the post-relocation heap size in SPL.

Fixes: dfc2dff5a844 ("configs: am62x_evm_*: Enable USB and DFU support")

Signed-off-by: Jonathan Humphreys <j-humphreys@ti.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>