atmel_serial: Fix build on avr32 with CONFIG_PM enabled

AVR32 doesn't have at91_suspend_entering_slow_clock(). Just assume the
clock will keep running for now.

David has a better solution for this, but this works for now. Leaving
the USART clock running won't prevent the PM code from entering deep
power-down modes anyway.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: David Brownell <david-b@pacbell.net>
Cc: Andrew Victor <linux@maxim.org.za>