drm/ast: Use constants for SDRAM registers

drm/ast: Use constants for SDRAM registers

Aspeed hardware allows for acceessing the SDRAM from the host. SDRAM
registers are located at the memory range at [0x80000000, 0xffffffff].

Refer to memory access with the macro AST_SDRAM(). Also add a TODO item
for the nonsensical documentation next to its caller.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260327133532.79696-9-tzimmermann@suse.de