platform/chrome: chromeos_pstore: set user space log size

On x86 ChromiumOS devices, the pmsg_size is set to 0 (check
/sys/module/ramoops/parameters/pmsg_size): this prevents use of
pstore-pmsg, even if CONFIG_PSTORE_PMSG is enabled. Set pmsg_size
to a value that is consistent with the size used on non-x86 ChromiumOS
devices.

Signed-off-by: Sarthak Kukreti <sarthakkukreti@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>