Libraries: Add include guard for SDRAM

[tokiedokie]

Include SDRAM.h twice, I get the following error

/home/tokiedokie/.arduino15/packages/arduino/hardware/mbed_portenta/4.0.2/libraries/Portenta_SDRAM/src/SDRAM.h:10:7: error: redefinition of 'class SDRAMClass'
 class SDRAMClass {
       ^~~~~~~~~~
In file included from /home/tokiedokie/.var/app/cc.arduino.IDE2/cache/.arduinoIDE-unsaved2024026-51-1ynz9ib.fpy2/sketch_jan26a/sketch_jan26a.ino:1:0:
/home/tokiedokie/.arduino15/packages/arduino/hardware/mbed_portenta/4.0.2/libraries/Portenta_SDRAM/src/SDRAM.h:10:7: note: previous definition of 'class SDRAMClass'
 class SDRAMClass {
       ^~~~~~~~~~

Adding an include guard can solve this problem.

fix: #808

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