board: toradex: verdin-am62_r5: Increase SPL_STACK_R_MALLOC_SIMPLE_LEN

Increase the SPL_STACK_R_MALLOC_SIMPLE_LEN to 0x200000 to accommodate the
size of tispl.bin fit image. With the recent upgrade of ti-linux-firmware
from version v9.1.0 to v9.2.5, the size of tispl.bin fit image has
increased to 1.4MB, causing allocation errors in the R5 SPL:

```
alloc space exhausted
Could not get FIT buffer of 1325056 bytes
        check CONFIG_SPL_SYS_MALLOC_SIZE
```

Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>