clk: at91: select parent if main oscillator or bypass is enabled

Selecting the right parent for the main clock is done using only
main oscillator enabled bit.
In case we have this oscillator bypassed by an external signal (no driving
on the XOUT line), we still use external clock, but with BYPASS bit set.
So, in this case we must select the same parent as before.
Create a macro that will select the right parent considering both bits from
the MOR register.
Use this macro when looking for the right parent.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lkml.kernel.org/r/1568042692-11784-2-git-send-email-eugen.hristev@microchip.com
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>