platform: generic: Don't mark non-MMU HARTs as invalid

Currently, the generic platform fw_platform_init() marks non-MMU
HARTs (e.g. E-core on SiFive Unleashed) as invalid. This means
such non-MMU HARTs won't be allowed to go ahead by sbi_init().

The sbi_init() now has a check for next stage privilege mode when
selecting coldboot HART. This check will force non-MMU HARTS (i.e.
HARTs without S-mode) to proceed in warmboot path and wait in the
HSM STOPPED state. This means we don't need to mark non-MMU HARTs
as invalid in generic platform fw_platform_init().

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