reset: ast2500: Use SCU for reset control

The System Control Unit (SCU) controller of Aspeed
SoCs provides the reset control for each peripheral.

This patch refactors the reset method to leverage
the SCU reset control. Thus the driver dependency
on watchdog including dedicated WDT API and reset
flag encoding can be eliminated.

The Kconfig description is also updated accordingly.

Signed-off-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Ryan Chen <ryan_chen@aspeedtech.com>