pwm: Let PWM_CLPS711X depend on HAS_IOMEM

PWM_CLPS711X needs HAS_IOMEM, so depend on it, the related error (with
allmodconfig under um):

    MODPOST 1205 modules
  ERROR: "devm_ioremap_resource" [drivers/pwm/pwm-clps711x.ko] undefined!
  ERROR: "devm_ioremap" [drivers/net/phy/mdio-bcm-unimac.ko] undefined!

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>