arm: socfpga: Move linux_qspi_enable from bootcommand to board_prep_linux function

Move 'linux_qspi_enable' from bootcommand to board_prep_linux function when
OS booted from FIT image for Stratix 10 and Agilex. This flow is common for
all Intel SOC64 devices.

U-Boot will update 'fdt_addr' environment value based on FIT image in
board_prep_linux function, and 'linux_qspi_enable' will refer to 'fdt_addr'
environment value to retrieve the device tree node.

Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>