arm: socfpga: scan: Introduce generic JTAG accessor

Introduce generic function for accessing the JTAG scan chains in the
SCC manager. Make use of this function throughout the SCC manager to
replace the ad-hoc writes to registers and make the code less cryptic.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Dinh Nguyen <dinguyen@opensource.altera.com>