x86: slimbootloader: Add memory configuration

Slim Bootloader provides memory map info thru its HOB list pointer.
Configure memory size and relocation memory from the HOB data, and
provide e820 entries as well.
- Get memory size from the memory map info HOB
- Set available top memory lower than 4GB for U-Boot relocation
- Provide e820 entries from the memory map info HOB

Signed-off-by: Aiden Park <aiden.park@intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>