add block write function to spartan3 slave serial load

Using seperate function calls for each bit-bang of slave serial
load can be painfully slow. This patch adds the possibility to
supply a block write function that loads the complete block of
data in one call (like it can already be done with Altera FPGAs).
On an MCF5373L (240 MHz) loading an XC3S4000 this reduces the load
time from around 15 seconds to around 3 seconds

Signed-off-by: Wolfgang Wegner <w.wegner at astro-kom.de>