ARM: Fix .size directive for xscale_dma_a0_map_area

gas used to accept (and ignore?) .size directives which referred to
undefined symbols, as this does.  In binutils 2.21 these are treated
as fatal errors.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>