rkcommon.c: Remove unused rkcommon_spi_to_offset

This function is unused, remove.  Reported by clang-3.8.

Fixes: a1c29d4b ("rockchip: mkimage: set init_boot_size to avoid ...")
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>