hwmon: (pmbus/adm1266) add powerup_counter debugfs entry

hwmon: (pmbus/adm1266) add powerup_counter debugfs entry

The ADM1266 maintains a 16-bit non-volatile POWERUP_COUNTER register
(0xE4, datasheet Rev. D, Table 93) that increments on every power
cycle and cannot be reset by the host. Each blackbox record already
embeds the counter at record time, so the standalone live value is
primarily useful for matching a captured record back to the boot it
came from when correlating logs.

Expose it as a read-only debugfs file alongside sequencer_state. The
block-read returns two payload bytes in little-endian order.

Take pmbus_lock around the block-read so the access serialises with
any pmbus_core sequence that sets PAGE on the device. Without it, a
PAGE write from another thread could interleave between a PAGE set
and a paged read elsewhere in the driver and corrupt either side's
view of the device state machine.

Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
Assisted-by: Claude-Code:claude-opus-4-7
Assisted-by: sashiko:gemini-3.1-pro-preview
Link: https://lore.kernel.org/r/20260520-adm1266-v5-2-c72ef1fac1ea@nexthop.ai
Signed-off-by: Guenter Roeck <linux@roeck-us.net>