soc: mediatek: reuse read[l,x]_poll_timeout helpers

Reuse the common helpers read[l,x]_poll_timeout provided by Linux core
instead of an open-coded handling. The name of the local variable
sram_pdn_ack in scpsys_power_on is renamed to pdn_ack in order to be
consistent with the one used in scpsys_power_off.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Weiyi Lu <weiyi.lu@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>