drm/ast: Rename ast_init_dram_2300 to ast_post_chip_2300

The function does more than initializing the DRAM and in turns
calls other functions to do the actual init. This will keeping
things more consistent with the upcoming AST2500 POST code.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Joel Stanley <joel@jms.id.au>
Tested-by: Y.C. Chen <yc_chen@aspeedtech.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>