arm: airoha: introduce AN7581 helpers to get SCU and CHIP_SCU regmaps

arm: airoha: introduce AN7581 helpers to get SCU and CHIP_SCU regmaps

We need access SCU and CHIP_SCU regmaps in several places (clk-airoha,
reset-airoha, airoha_eth). Unfortunately these regmaps can't be easily
retrieved with a common code, because of different Airoha SoCs uses
a different dts structure.

To make life easy we can write a commonly named SoC specific helpers
for these tasks. This patch implements helpers for Airoha AN7581 SoC.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>