arm: rmobile: alt: Remove dram_init_banksize function

Alt board is defined CONFIG_SYS_SDRAM_BASE, and this can use a basic function of
dram_init_banksize instead.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>