xen/arm: grant-table: Increased the initial number of grant frame to 4

xen/arm: grant-table: Increased the initial number of grant frame to 4

When a domain is created on ARM, the grant table code initialized one
grant frame. With a basic load (i.e disk usage), Xen is quickly trying
to expand the number of frames:

(XEN) grant_table.c:1305:d2v0 Expanding dom (2) grant table from (1) to (2) frames.
(XEN) grant_table.c:1305:d2v0 Expanding dom (2) grant table from (2) to (3) frames.
(XEN) grant_table.c:311:d0v0 Increased maptrack size to 2 frames
(XEN) grant_table.c:311:d0v0 Increased maptrack size to 3 frames

The x86 code is initialied 4 frames (I didn't find the exact reason). I
think we could use the same default value.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>