[ARM] vfp: fix fuitod/fsitod instructions

These two instructions exceptionally take a single precision register
as their operand.  This means we can't use vfp_get_dm() to read the
register number - we need to use vfp_get_sm() instead.  Add a flag to
indicate this exception to the general rule.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>