config: u-boot: spl-da: fix fastboot buf addr|size

With U-Boot SPL as Download Agent (DA), the fastboot buf address and size must be:

• FASTBOOT_BUF_ADDR: 0x65000000
• FASTBOOT_BUF_SIZE: 0x15000000

Fixes: 7ecc82f6 ("uboot: rework defconfig and fragments usage")

RITA-1248

Cc @eblanc @vsatoes