ppc4xx_sdram: Rename local state variable for brevity

ppc4xx_sdram: Rename local state variable for brevity

Rename the sdram local state variable to s in dcr read/write functions
and reset methods for better readability and to match realize methods.
Other places not converted will be changed or removed in subsequent
patches.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <8e7539cb1fccd7556b68351c4dcf62534c3a69cf.1666194485.git.balaton@eik.bme.hu>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>