configs: synquacer: increase SYS_MALLOC_F_LEN

DM_FLAG_PRE_RELOC flag is added into some drivers
by recent commits such as
1bd790bc ("firmware: psci: enable DM_FLAG_PRE_RELOC").
Current SYS_MALLOC_F_LEN of SynQuacer Developerbox platform
is too small, Developerbox will not boot due to lack of
heap memory.

This commit increases the size of heap memory.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>