misc: atmel_pwm: fix Kconfig symbols

AT91 symbols AT91SAM9263, AT91SAM9RL, and AT91SAM9G45 do not exist and this
patch changes them to their correct ARCH_* version.
These symbols are chosen instead of the SOC_* ones because this driver is not
converted to DT.
Anyway, the ATMEL_PWM symbol and the associated driver will be removed soon,
during the move to the PWM sub-system.

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>