KEYS: trusted: Add generic trusted keys framework

Current trusted keys framework is tightly coupled to use TPM device as
an underlying implementation which makes it difficult for implementations
like Trusted Execution Environment (TEE) etc. to provide trusted keys
support in case platform doesn't posses a TPM device.

Add a generic trusted keys framework where underlying implementations
can be easily plugged in. Create struct trusted_key_ops to achieve this,
which contains necessary functions of a backend.

Also, define a module parameter in order to select a particular trust
source in case a platform support multiple trust sources. In case its
not specified then implementation itetrates through trust sources list
starting with TPM and assign the first trust source as a backend which
has initiazed successfully during iteration.

Note that current implementation only supports a single trust source at
runtime which is either selectable at compile time or during boot via
aforementioned module parameter.

Suggested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -5462,6 +5462,18 @@
 			See Documentation/admin-guide/mm/transhuge.rst
 			for more details.
 
+	trusted.source=	[KEYS]
+			Format: <string>
+			This parameter identifies the trust source as a backend
+			for trusted keys implementation. Supported trust
+			sources:
+			- "tpm"
+			- "tee"
+			If not specified then it defaults to iterating through
+			the trust source list starting with TPM and assigns the
+			first trust source as a backend which is initialized
+			successfully during iteration.
+
 	tsc=		Disable clocksource stability checks for TSC.
 			Format: <string>
 			[x86] reliable: mark tsc clocksource as reliable, this