hw/arm/realview.c: Fix memory leak in realview_init()

Variable 'ram_lo' is allocated unconditionally, but used only in some cases.
When it is unused pointer will be lost at function exit, resulting in a
memory leak. Allocate memory for 'ram_lo' only if it is needed.

Valgrind output:
==16879== 240 bytes in 1 blocks are definitely lost in loss record 6,033 of 7,018
==16879==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16879==    by 0x33D2CE: malloc_and_trace (vl.c:2804)
==16879==    by 0x509E610: g_malloc (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
==16879==    by 0x288836: realview_init (realview.c:55)
==16879==    by 0x28988C: realview_pb_a8_init (realview.c:375)
==16879==    by 0x341426: main (vl.c:4413)

Signed-off-by: Nikita Belov <zodiac@ispras.ru>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
