x86/virt/tdx: Move mk_keyed_paddr() to tdx.c due to no external users

x86/virt/tdx: Move mk_keyed_paddr() to tdx.c due to no external users

Move mk_keyed_paddr() from tdx.h to tdx.c to avoid unnecessary header
inclusion and improve encapsulation since there are no users outside of
tdx.c.

No functional change intended.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Acked-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20260430015014.24261-1-yan.y.zhao@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>