firmware: Don't depend on PLAT_HART_COUNT and PLAT_HART_STACK_SIZE

The hart_count and hart_stack_size information is already available
in "struct sbi_platform" so we use that instead of depending on
PLAT_HART_COUNT and PLAT_HART_STACK_SIZE.

Signed-off-by: Anup Patel <anup.patel@wdc.com>