rockchip: spl: do full dram_init instead of only probing

Parts of later SPL may need RAM information as well, so do full
dram_init() call, which includes the existing dram probing but also
initializes the ram information in gd.

dram_init() from sdram.c does the following steps:
- uclass_get_device(UCLASS_RAM, ...) like the current code
- ret = ram_get_info(dev, &ram);
- gd->ram_size = ram.size;

CONFIG_SPL_RAM already makes sure that sdram.c gets compiled
and thus no other variant of dram_init() can exist.

So it's the same functionality as before and only adds that the
SPL now aquires knowledge about the amount of available ram,
which it didn't know about before.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>