platform: sifive/fu540: Replace cpu0 node fix-up with the new helper

This replaces the FU540 specific cpu0 node "status" property fix-up
with the newly introduced generic fdt_cpu_fixup() helper.

Unlike previous logic, the helper routine does not test the "mmu-type"
property to determine which node we should fix up, instead it uses
sbi_platform_hart_disabled() API.

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