firmware: smccc: Update link to latest SMCCC specification

The current link gets redirected to the revision B published in November
2016 though it actually points to the original revision A published in
June 2013.

Let us update the link to point to the latest version, so that it
doesn't get stale anytime soon. Currently it points to v1.2 published in
March 2020(i.e. DEN0028C).

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Etienne Carriere <etienne.carriere@st.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Etienne Carriere <etienne.carriere@st.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20200518091222.27467-3-sudeep.holla@arm.com


Signed-off-by: Will Deacon <will@kernel.org>