riscv: sifive: fu540: Sync-up config header with RISC-V QEMU support

We typically use same set of distro images (yocto, debian, fedora, etc.)
on both QEMU RISC-V virt machine and SiFive Unleashed board.

With growing kernel and ramdisk images, we need to re-adjust default
U-Boot environment variables. The config header for QEMU RISC-V virt
machine has been already updated to handle bigger kernel and ramdisk
images hence this patch updates SiFive FU540 config header accordingly.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Tested-by: David Abdurachmanov <david.abdurachmanov@sifive.com>