clk: allow a clk divider with max divisor when zero

This commit allows certain Broadcom STB clock dividers to be used with
clk-divider.c.  It allows for a clock whose field value is the equal
to the divisor, execpt when the field value is zero, in which case the
divisor is 2^width.  For example, consider a divisor clock with a two
bit field:

value		divisor
0		4
1		1
2		2
3		3

Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>