firmware: arm_sdei: Drop check for /firmware/ node and always register driver

firmware: arm_sdei: Drop check for /firmware/ node and always register driver

As with most of the drivers, let us register this driver unconditionally
by dropping the checks for presence of firmware nodes(DT) or entries(ACPI).

Further, as mentioned in the commit acafce48b07b ("firmware: arm_sdei:
Fix DT platform device creation"), the core takes care of creation of
platform device when the appropriate device node is found and probe
is called accordingly.

Let us check only for the presence of ACPI firmware entry before creating
the platform device and flag warning if we fail.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
Cc: James Morse <james.morse@arm.com>
Link: https://lore.kernel.org/r/20200422122823.1390-1-sudeep.holla@arm.com
Signed-off-by: Will Deacon <will@kernel.org>