soc: adi: max32: Add. shared section for MAX7800X SoC

The SystemCoreClock variable must be placed in the '.shared' section for
the MAX78000 and MAX78002 because the ARM core is responsible for changing
the System Clock and updating the SystemCoreClock variable, and the
RISC-V core knows what the System Clock frequency is through the
shared variable.

Signed-off-by: Yasin Ustuner <Yasin.Ustuner@analog.com>