Some platforms that can be supported with this driver have PRTCON
register instead of SHADOWCON, which requires slightly different
handling.

This patch factors out all register shadow control code from the driver
and adds a function to control register shadowing appropriately,
depending on driver data.
Signed-off-by: NTomasz Figa <tomasz.figa@gmail.com>
Acked-by: NJoonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: NInki Dae <inki.dae@samsung.com>