[ARM] 2907/1: GCC 4 serial driver compile fixes

Patch from Vincent Sanders

When building the ARM platforms several serial drivers fail to compile
with GCC 4.01 due to extern/static ambiguity.

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