Claude review: drm/panthor: Use a local iomem base for PWR registers

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Adds `pwr->iomem = ptdev->iomem + PWR_CONTROL_BASE`. Converts all PWR register accesses. The `PWR_CTRL_REG()` macro is removed and all PWR register offsets become relative to the block base.

**Minor**: The commit message has a typo: "PWR_CONTROl-local" (lowercase 'l' in CONTROL).

**Note**: This patch is missing Reviewed-by/Acked-by tags (only has Signed-off-by), presumably because it was significantly reworked in v3.

**Pre-existing issue** (not introduced by this series): In `panthor_pwr_l2_power_on()`:
```c
const u32 pwr_status = gpu_read64(pwr->iomem, PWR_STATUS);
const u32 l2_allow_mask = PWR_STATUS_DOMAIN_ALLOWED(PWR_COMMAND_DOMAIN_L2);
```
Both are `u32` but `PWR_STATUS` is a 64-bit register and `PWR_STATUS_DOMAIN_ALLOWED()` uses `BIT_U64()`. The truncation happens to be harmless for the L2 domain (bit 0), but should probably be `u64` for correctness. Not something this series needs to fix, but worth noting.

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