The ARISC vector area consists of 0x4000 bytes before the beginning of
usable SRAM. Still, it is technically a part of SRAM A2, so include it
in the memory definition. This avoids the confusing practice of
subtracting from the beginning of the SRAM region when referencing the
ARISC vectors.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Change-Id: Iae89e01aeab93560159562692e03e88306e2a1bf