layout_riscv32.ld: Update the SRAM size to be correct
layout_riscv32.ld: Update the SRAM size to be correct

Reduce the size so we don't go over the memory avaliable in the hardware
or QEMU.

Signed-off-by: Alistair Francis <52ecf00b70a50079ae8d5fa09b9af6904950b63c@wdc.com>

tock/libtock-rs