backends/arm: Bump ITCM mem to 1MB in Corstone 300 linker script (#3830)

Summary:
This fix linker errors when building for Corestone 300 caused by code size got slightly bigger and didn't fit the linker script setup. When using run.sh Corstone 300 was already started with the needed larger ITCM memory size.

Pull Request resolved: https://github.com/pytorch/executorch/pull/3830

Reviewed By: mergennachin

Differential Revision: D58138173

Pulled By: digantdesai

fbshipit-source-id: 34a88d0dfc34c380ac7ca90b3fcfd68012d10f7c