cxl/core: Remove duplicate declaration of devm_cxl_iomap_block()

The function devm_cxl_iomap_block() is only used in the core
code. There are two declarations in header files of it, in
drivers/cxl/core/core.h and drivers/cxl/cxl.h. Remove its unused
declaration in drivers/cxl/cxl.h.

Fixing build error in regs.c found by kernel test robot by including
"core.h" there.

Signed-off-by: Robert Richter <rrichter@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20221018132341.76259-2-rrichter@amd.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>