spi: bcm63xx_hsspi: Continue init when using no reset and fixed-clock.

The Broadcom ARM implementations do not yet have a clock framework so
one can use a fixed clock as the root clock of the hsspi block. The
fixed clock does not have an "enable" routine, since it's always
enabled. So when we hit this issue, getting an ENOSYS return, do not
bail but continue initialization.

Similarly the block might already have been out of reset, say, when
we are booting from a SPI device. So if the reset signal is not configured
in the device tree, do not bail out and instead skip deasserting the reset.

Signed-off-by: Kursad Oney <kursad.oney@broadcom.com>
Reviewed-by: Philippe Reynes <philippe.reynes@softathome.com>