efm32gg990f1024: added linker script

lib/efm32/efm32gg/libopencm3_efm32gg990f1024.ld

@@ -0,0 +1,15 @@
+/* lengths from d046_efm32gg990_datasheet.pdf table 1.1, offset from
+ * d0034_efm32tg_reference_manual.pdf figure 5.2.
+ *
+ * the origins and memory structure are constant over all giantgeckos, but the
+ * MEMORY section requires the use of constants, and has thus to be duplicated
+ * over the chip variants.
+ * */
+
+MEMORY
+{
+	rom (rx) : ORIGIN = 0, LENGTH = 1024k
+	ram (rwx) : ORIGIN = 0x20000000, LENGTH = 128k
+}
+
+INCLUDE libopencm3_efm32gg.ld;