s390/sclp: use memblock for early read cpu info

sclp early read cpu info is used to detect the number of configured
cpus, which is utilized by smp_detect_cpus() in early startup.

* For read cpu info, the sccb block should be below 2gb.
* smp_detect_cpus() utilizes read cpu info early, but after memblock
  initialization. Thus use memblock_allow_low() instead.
* Avoid copy of sclp_core_info structure.
* sclp_early_init_core_info(), sclp_early_core_info and
  sclp_early_core_info_valid initdata are no longer required.
* smp_get_core_info() is called only once during early stage.  Hence for
  early sclp_get_core_info(), directly call read cpu command. No need to
  maintain sclp_early_core_info_valid.

Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>