Improve format of exception vectors in BL1 description

Without the additional newlines all the text becomes a single paragraph
and next newlines are ignored.

Change-Id: I783198477f654e3923fcabb21248f2bc62c33e9d
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>

docs/firmware-design.md

@@ -138,6 +138,7 @@ BL1 performs minimal architectural initialization as follows.
     code is one of:
 
     For AArch64:
+
         0x0 : Synchronous exception from Current EL with SP_EL0
         0x1 : IRQ exception from Current EL with SP_EL0
         0x2 : FIQ exception from Current EL with SP_EL0
@@ -156,6 +157,7 @@ BL1 performs minimal architectural initialization as follows.
         0xf : System Error exception from Lower EL using aarch32
 
     For AArch32:
+
         0x10 : User mode
         0x11 : FIQ mode
         0x12 : IRQ mode