ARM: at91: fix at91_suspend_entering_slow_clock link error

ARM: at91: fix at91_suspend_entering_slow_clock link error

When CONFIG_ARCH_AT91 is enabled, but none of the specific SoC support
is in use, some at91 specific drivers fail to link:

drivers/tty/serial/atmel_serial.o: In function `atmel_serial_suspend':
atmel_serial.c:(.text.atmel_serial_suspend+0x1e): undefined reference to `at91_suspend_entering_slow_clock'
drivers/usb/host/ohci-at91.o: In function `ohci_hcd_at91_drv_suspend':
ohci-at91.c:(.text.ohci_hcd_at91_drv_suspend+0x12): undefined reference to `at91_suspend_entering_slow_clock'
drivers/usb/gadget/udc/at91_udc.o: In function `at91udc_suspend':
at91_udc.c:(.text.at91udc_suspend+0x26): undefined reference to `at91_suspend_entering_slow_clock'

This changes the at91_suspend_entering_slow_clock hack once more, adding
an alternative inline implementation that is used exactly in those cases
that don't provide the normal implementation.

Fixes: c1892c2379d2 ("ARM: at91: handle CONFIG_PM for armv7m configurations")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>