staging: ion: optimize struct ion_system_heap

struct ion_system_heap has an array for storing pointers to page pools
and it is allocated separately from the containing structure. There is
no point in allocating those two small objects individually, bothering
slab allocator. Using a variable length array simplifies code lines and
reduces overhead to the slab.

Signed-off-by: Heesub Shin <heesub.shin@samsung.com>
Reviewed-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>