sunxi: Fix pll1 clock calculation

clock_sun6i.c is used for sun6i, sun8i and sun50i SoC families.
PLL1 clock sets the default system clock, defined as:
  sun6i: 1008000000
  sun8i: 1008000000
  sun50i: 816000000

With the current calculation, m = 2 and k = 3. Solving for n,
this results 28. Solving back:
  (24MHz * 28 * 3) / 2 = 1008MHz

However if the requested clock is 816, n is 22.66 rounded
to 22, which results:
  (24MHz * 28 * 3) / 2 = 792MHz

Changing k to 4 satisfies both system clocks:
  (24E6 * 21 * 4) / 2 = 1008MHz
  (24E6 * 17 * 4) / 2 = 816MHz

Signed-off-by: Stefan Mavrodiev <stefan@olimex.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>