qemu-riscv: Fix 'Discovered' DDR size

qemu-riscv value for DDR is not correct in how it was being used in code.
It was set to 32768 which in turn is being used as 32768MB (32+GB).  This
is causing an expection when trying to access 32GiB boundary when only 1GiB
is present in the system

Change value so it is a select option based in menuconfig
based on what you want coreboot to use on boot.

Four options are :
256MB
512MB
1024MB
2048MB

NOTE:  You must specify greater than or equal to what is
chosen from the above items on the qemu cmdline.

This value is used in both romstage and ramstage to set the amount
of DDR present.  This will be the above value minus the size of the
boot rom.

Updated Documentation for above change
Fixes: https://ticket.coreboot.org/issues/254

TEST = qemu-system-riscv64 -M virt -m 1024M -nographic \
	-bios build/coreboot.rom

TEST = qemu-system-riscv64 -M virt -m 1024M -nographic \
	-kernel build/coreboot.elf

Signed-off-by: Marc Karasek <mkarasek@cryptocoretech.com>
Change-Id: Idef44eb8baf3e89d7c74fa452fc60b7beefa2c48
---
M Documentation/mainboard/emulation/qemu-riscv.md
M src/mainboard/emulation/qemu-riscv/Kconfig
M src/mainboard/emulation/qemu-riscv/mainboard.c
M src/soc/ucb/riscv/cbmem.c
4 files changed, 47 insertions(+), 6 deletions(-)