ARC: allow selection of MMU page size

Modern ARC cores (those sporting MMU of version 3 and 4) allow selection
of different page sizes (4, 8 or 16 kB) during ASIC design creation.
And it's important to build a toolchain with page size setting that matches
hardware.

Otherwise user-space applications will fail on execution due to
unexpected data layout/alignment etc.

[Thomas: slightly improve help text, fix indentation of help text.]

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Anton Kolesov <akolesov@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>