spi_qsd: Add high speed mode support

The SPI mini-core has a specialized signal sampling mode which
improves signal to noise ratio for bus rates that are greater
then or equal to 26MHz.
This patch sets the mini-core in high-speed mode whan
the bus rate is greater or equal to this limit.

CRs-Fixed: 460451
Change-Id: I9a5094c92f3be8b683cd28a7737501d26b45b082
Signed-off-by: Gilad Avidov <gavidov@codeaurora.org>