powercap: intel_rapl: Use unit conversion macros from units.h

powercap: intel_rapl: Use unit conversion macros from units.h

Replace hardcoded numeric constants with standard unit conversion
macros from linux/units.h for better code clarity and
self-documentation.

Add MICROJOULE_PER_JOULE and NANOJOULE_PER_JOULE to units.h to
support energy unit conversions, following the existing pattern
for power units.

No functional changes.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Link: https://patch.msgid.link/20260212233044.329790-8-sathyanarayanan.kuppuswamy@linux.intel.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>