platform: fu540: Provide a compile time option selective hart booting.

Currently, only hart 1 is enabled for fu540 platform to support U-boot.

Introduce a compile time FU540_ENABLED_HART_MASK option so that specific
harts can be selected for booting. As fu540 is a multicore, we should
boot all cores by default except hart 0.

Signed-off-by: Atish Patra <atish.patra@wdc.com>