x86: acpi: Return table length in acpi_create_madt_lapics()

Like other MADT table write routines, make acpi_create_madt_lapics()
return how many bytes it has written instead of the table end addr.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>