riscv: Enable CONFIG_SYS_BOOT_RAMDISK_HIGH for using initrd

riscv: Enable CONFIG_SYS_BOOT_RAMDISK_HIGH for using initrd

This patch enables CONFIG_SYS_BOOT_RAMDISK_HIGH for RISC-V
because bootm will update initrd location in DTB only if
CONFIG_SYS_BOOT_RAMDISK_HIGH is enabled. If we don't enable
this option then bootm assumes DTB already has initrd details
which is not the case most of the time.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>