apm821xx: MBL: HDD regulators overhaul for MBL DUO

apm821xx: MBL: HDD regulators overhaul for MBL DUO

Takimata reported on the OpenWrt forum in thread [0], that his
MyBook Live Duo wasn't booting OpenWrt 21.02 after upgrading
from the previous OpenWrt 19.07.

The last logged entries on his console

|[    0.531599] sata1-regulator GPIO handle specifies active low - ignored
|[    0.538391] sata0-regulator GPIO handle specifies active low - ignored
|[    0.759791] ata2: SATA link down (SStatus 0 SControl 300)
|[    0.765251] ata1: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
|[    5.909555] ata1.00: qc timeout (cmd 0xec)
|[    5.913656] ata1.00: failed to IDENTIFY (I/O error, err_mask=0x4)
|[    6.231757] ata1: SATA link down (SStatus 0 SControl 300)

This extract clearly showed that the HDD on which OpenWrt is installed,
simply disappeared after the SATA power regulators had been initialized.

The reason why this worked with OpenWrt 19.07 was because the kernel
config symbol CONFIG_REGULATOR=y was not set in the target's config-4.14.

(This shows that the MBL Single does differ from the DUO in that
it does not have programmable power regulators for the HDDs.)

[0] <https://forum.openwrt.org/t/21-02-0-and-snapshot-fail-to-boot-on-my-book-live-duo/106585>

Reported-by: Takimata (forum)
Tested-by: Takimata (forum)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>