spi: add Kconfig entries for SPI4 and SPI5.

The SAM0 series has up to 6 SPI ports.  Add Kconfig options to match.

Similar to the 9033fb2f, this patch
only defines the fields that are currently used and skips ones like
GPIO and IRQ priority that aren't.

Signed-off-by: Michael Hope <mlhx@google.com>