mips: mtmips: Increase CONFIG_SPL_SYS_MALLOC_F_LEN

Since SPL now uses malloc to allocate the buffer for the compressed
image before decompression to the destination address, we need to
configure a big enough malloc space in SPL. 256k seems to be big
enough even for the GARDENA board.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Weijie Gao <weijie.gao@mediatek.com>