target-arm: Convert generic timer reginfo to accessfn

target-arm: Convert generic timer reginfo to accessfn

Convert the reginfo structs for the generic timer registers
to use access functions rather than returning EXCP_UDEF from
their read handlers. In some cases this allows us to remove
a read handler completely.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>