soc: esp32c6: add userspace linker support

Add linker script support for CONFIG_USERSPACE:
- MPU alignment macros for PMP granularity
- User stacks section in noinit area
- Application shared memory partitions
- Kernel object sections (text, rom, data, priv-stacks)
- ROM region size symbol for PMP configuration

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>