coresight: Simplify sysfs accessors using common struct to avoid passing the type

All of the sysfs read callbacks pass structs that have the 'base'
member as the first entry of the struct. As long as this is true,
it's not necessary to pass in the type to the macro.

In the next change, the register will be passed in as a value
rather than creating new functions for each accessor, and it's
not possible to pass in the struct type in this way so just drop
it.

static_asserts ensure that the member isn't moved in the future.

No functional changes.

Signed-off-by: James Clark <james.clark@arm.com>