ARM: 6376/1: plat-nomadik: MTU: Change prescaler limit and comment updates

The prescaler 16 is now used only when the timer runs at 32 MHz
or more. Some comment updates as well.

Acked-by: Alessandro Rubini <rubini@unipv.it>
Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>