configs: meson64_android: increase SYS_MALLOC_LEN to 128M for AVB

To prepare for AVB support, increase SYS_MALLOC_LEN to 128M.
This value has been found by testing the following on khadas vim3l:
  => avb init
  => avb verify

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>