riscv: set CONFIG_SYS_BOOTM_LEN to SZ_64M

After updating Fedora/RISCV kernel to 5.1-rc3+ the size increased above
the current threshold. Looking into HiKey, Dragonboards, etc. seems that
SZ_64M is a popular option.

This sucessfully boots Fedora/RISCV with 5.1-rc3+ kernel on QEMU 4.0 (master)
with OpenSBI -> U-Boot (S-mode) [extlinux] -> Kernel setup.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>