drm/ast: Move 32-bit register-access helpers to ast_drv.{c, h}

The helpers ast_mindwm() and ast_moutdwm() access the I/O memory of
the various IP modules on the Aspeed device. This is based on the
"P-Bus to AHB Bridge" interface.

Reimplement the access function with properly defined constants and
helper macros.

- Define P2A constants for the related registers and addresses. The P2A
interface is located in the memory range at [0x00000000, 0x00010000].

- Memory access is segmented. An address' upper 16-bit select the
memory segment, the lower 16-bit select the offset within the segment.
Implement segment selection in a shared helper __ast_segsel(). Validate
that the segment hs been changes. This logic has previously been part
of __ast_moudwm() and __ast_mindwm(). Relax the CPU while busy-waiting.

- Put intra-segment reads and writes in the helpers __ast_rdseg32()
and __ast_wrseg32(). The helpers set the segment offset automatically.

- Reimplement the existing interfaces on top of these helpers.

Put the new implementation next to the other I/O helpers.

v2:
- fix typo in commit description (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260327133532.79696-3-tzimmermann@suse.de

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