kernel: add (disabled) ASYMMETRIC_TPM_KEY_SUBTYPE symbol

kernel: add (disabled) ASYMMETRIC_TPM_KEY_SUBTYPE symbol

at91/sama7 fails to build due to:

| Asymmetric (public-key cryptographic) key type (ASYMMETRIC_KEY_TYPE) [Y/?] y
|  Asymmetric public-key crypto algorithm subtype (ASYMMETRIC_PUBLIC_KEY_SUBTYPE) [Y/?] y
|  Asymmetric TPM backed private key subtype (ASYMMETRIC_TPM_KEY_SUBTYPE) [N/m/?] (NEW)
|Error in reading or end of file.

please note that asym_tpm (module) has been removed in 5.17:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d3cff4a9>

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>