quark_d2000: Add shared GDT memory to linker

A new shared memory area has been added to the QMSI bootloader in
order to store the GDT in RAM reliably.

Add the new entry to the QUARK D2000 linker script and new
kconfig options:

 * CONFIG_BSP_SHARED_GDT_RAM_ADDR to set the address location
   of the GDT in RAM.

 * CONFIG_BSP_SHARED_GDT_RAM_SIZE to set the size used by the
   GDT in RAM.

This is only enabled when  CONFIG_SET_GDT is NOT set.

Change-Id: Ie084079475853dce153dd9f3e88ad4a9784bdedf
Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>