platform/x86: intel_pmc_core: Remove unused header file

Recently sent patch 'platform/x86: intel_pmc_core: Remove unused EXPORTED
API' missed to remove the header file 'arch/x86/include/asm/pmc_core.h'
which was solely used to declare the EXPORTED API
'intel_pmc_slp_s0_counter_read'. This patch provides the errata fix for the
same.

Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>