ARM: at91: remove dependency for Atmel PWM driver selector in Kconfig

Remove dependency line which was not including the SAM9G45.

[arnd: added dependency on HAVE_CLK to avoid breaking other machines]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>