configs: meson64: permit redefining SYS_MALLOC_LEN

Permit redefining SYS_MALLOC_LEN for board specific configs.
This is especially useful for Android with AVB, which requires a malloc
length of 128M.

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