arch/tile: add some more VMSPLIT options and use consistent naming

This renames 3G_OPT to 2_75G, and adds 2_5G and 2_25G.

For memory-intensive applications that are also network-buffer
intensive it can be helpful to be able to tune the virtual address
of the start of kernel memory.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>