zynq: Kconfig: extend the bootstrap malloc() pool

Most of the memory is being consumed by device binding code,
more space needed for other data structures.

Z-turn board has already hit the limit, others may follow soon.

Measuring only the memory consumed in device_bind_common, I've got
the following results (in decimal):

  root_driver:               108
  mod_exp_sw:                108
  amba:                      120
  serial@e0000000 aka uart0: 112
  serial@e0001000 aka uart1: 88
  spi@e000d000 aka qspi:     120
  sdhci@e0100000 aka mmc0:   455
  sdhci@e0100000.blk:        208
  slcr@f8000000:             96
  clkc@100:                  72
  (total)                    1487 = 0x5cf of 0x600

Signed-off-by: Anton Gerasimov <tossel@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>