configs: j721s2_evm_r5_defconfig: Increase malloc pool size in DRAM

The malloc capacity in DRAM at R5 SPL is set to 1MB which isn't
sufficient to load the new tispl.bin to
enable loading of tispl.bin the size is increased by 256KB to 1.25MB.

Cc: Nikhil M Jain <n-jain1@ti.com>
Signed-off-by: Udit Kumar <u-kumar1@ti.com>
Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Reviewed-by: Nikhil M Jain <n-jain1@ti.com>