platform/x86: intel_pmc_ipc: Use spin_lock to protect GCR updates

Currently, update_no_reboot_bit() function implemented in this driver
uses mutex_lock() to protect its register updates. But this function is
called with in atomic context in iTCO_wdt_start() and iTCO_wdt_stop()
functions in iTCO_wdt.c driver, which in turn causes "sleeping into
atomic context" issue. This patch fixes this issue by replacing the
mutex_lock() with spin_lock() to protect the GCR read/write/update APIs.

Fixes: 9d855d46 ("platform/x86: intel_pmc_ipc: Fix iTCO_wdt GCS memory mapping failure")
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kupuswamy@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>