mach-sc5xx: Rename SC_BOOT_MODE

mach-sc5xx: Rename SC_BOOT_MODE

The symbol SC_BOOT_MODE was named incorrectly and inconsistently with
its usage. The selected boot mode is set only by hardware and cannot be
adjusted through software (apart from the use of FORCE_BMODE to instruct
the boot rom to ignore the hardware setting when loading uboot proper,
but this cannot change how SPL is loaded).

This symbol actually controlled the BCODE (easily confused with BMODE,
shorthand for boot mode), so this renames it to SC_BCODE and updates the
help text to reflect its actual usage: the BCODE is an SoC- and boot
mode-specific setting that affects how the boot rom configures QSPI or
OSPI in order to read an LDR file from the associated peripheral.

Reported-by: Philip Molloy <Philip.Molloy@analog.com>
Signed-off-by: Greg Malysa <malysagreg@gmail.com>