drm/ast: Use constants for MCR registers

drm/ast: Use constants for MCR registers

SDRAM registers are located in the memory range at
[0x1e160000, 0x1e160fff]. Refer to them with constants named
AST_REG_MCR<n>, where n is the byte offset into the range.

Replacing the magic values in the ast driver was done with grep
and sed as shown below

  git grep -l \,\ 0x1e6e00 | xargs sed -i -e 's/, 0x1e6e00/, AST_REG_MCR/g'
  git grep -l \,\ 0x1E6E00 | xargs sed -i -e 's/, 0x1E6E00/, AST_REG_MCR/g'

plus some manual fixes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260327133532.79696-5-tzimmermann@suse.de