platform/x86/amd/hsmp: Use IS_ENABLED() instead of IS_REACHABLE()

platform/x86/amd/hsmp: Use IS_ENABLED() instead of IS_REACHABLE()

IS_REACHABLE() was required when CONFIG_HWMON was set to m and HSMP to y.
However, commit 69157b00b526 ("platform/x86/amd/hsmp: fix building with
CONFIG_HWMON=m") added a HWMON dependency for HSMP in Kconfig. With
this change, using IS_ENABLED() is sufficient.

Add the missing header file as well.

Signed-off-by: Suma Hegde <suma.hegde@amd.com>
Link: https://lore.kernel.org/r/20250603055807.2503028-1-suma.hegde@amd.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>