platform: sifive/fu540: Remove FU540_ENABLED_HART_MASK option

The FU540_ENABLED_HART_MASK compile time option was added for initial
bring-up on SiFive Unleashed. This option is redundant now because
disabled_hart_mask is already removed. Based on this rationale, we
remove FU540_ENABLED_HART_MASK compile time option.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>