environment: ti: k3_dfu: Increase the size allocated for bootloader images in dfu_alt_info_ram

The size of u-boot.img is above 1MB and that of tispl.bin is close to 1MB,
in case of j721e. Therefore, increase the sizes allocated for tispl.bin and
u-boot.img to 2 MB and 4 MB respectively, in dfu_alt_info_ram environment
variable.

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