am335x: memory configuration

[Yegor Yefremov]

I've submitted a patch [1] to support our Baltos systems back in the
May. There was an issue with dynamically detecting RAM size as the
systems can have either 256MB or 512MB modules.

So far u-boot was able to detect the RAM size dynamically. In order to
do so it uses following code in arch/arm/cpu/armv7/am33xx/emif4.c:

int dram_init(void)
{

#ifndef CONFIG_SKIP_LOWLEVEL_INIT
        sdram_init();
#endif

        /* dram_init must store complete ramsize in gd->ram_size */
        gd->ram_size = get_ram_size(
                        (void *)CONFIG_SYS_SDRAM_BASE,
                        CONFIG_MAX_RAM_BANK_SIZE);
        return 0;
}

void dram_init_banksize(void)
{
        gd->bd->bi_dram[0].start = CONFIG_SYS_SDRAM_BASE;
        gd->bd->bi_dram[0].size = gd->ram_size;
}

This peace of code will be excuted in u-boot.img (not in MLO).

In Barebox we have get_ram_size(), that provides real RAM size and
am335x_sdram_size(), that returns RAM controller settings. So far DDR
controler will be configured for 512MB regardless of the module. Hence
the return value from am335x_sdram_size() cannot be used for crating
ram0 node.

What were the best way to provide get_ram_size() from lowlevel.c to
board.c, where I would invoke:
arm_add_mem_device("ram0", 0x80000000, sdram_size);

[1] http://lists.infradead.org/pipermail/barebox/2016-May/027224.html

Yegor