ARM: keystone2: use detected ddr3a size

ARM: keystone2: use detected ddr3a size

Because KS2 u-boot works in 32 bit address space the existing ram_size
global data field cannot be used. The maximum, which the get_ram_size()
can detect is 2GB only. The ft_board_setup() needs the actual ddr3 size
to fix up dtb.

This commit introduces the ddr3_get_size() which uses SPD data to
calculate the ddr3 size. This function replaces the "ddr3_size"
environment variable, which was used to get the SODIMM size.

For platforms, which don't have SODIMM with SPD and ddr3 is populated to
a board a simple ddr3_get_size function that returns ddr3 size has to be
implemented. See hardware-k2l.h

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>