armv7: ls102xa: make QSPI clock selection optional during SoC init

armv7: ls102xa: make QSPI clock selection optional during SoC init

To improve startup times when booting from QSPI flash, the QSPI frequency
can be configured very early in the boot process [1] to reduce loading
times of U-Boot itself. This patch adds an option to disable setting the
frequency to a default value during SoC initialization.

[1] https://www.nxp.com/docs/en/application-note/AN12279.pdf

Signed-off-by: Mario Kicherer <dev@kicherer.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>