memory: renesas-rpc-if: Simplify single/double data register access

For manual write and read, factor out the common access to the first
data register by keeping track of the current data pointer.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/c3b2a8d1a69f1b1e8d1a460148406cfb83e52eb4.1649857740.git.geert+renesas@glider.be


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>