[PARISC] Simplify DISCONTIGMEM in Kconfig

parisc was previously displaying both the mm/Kconfig memory model
menu, and its own prompt. Remove prompt and have CONFIG_64BIT toggle
between DISCONTIGMEM and FLATMEM. Also remove the EXPERIMENTAL from
discontigmem support... It's been running fine for months (years?)
now.

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>