arm64: dts: r8a7795: Correct SATA device size to 2MiB

Update the r8a7795 SATA device node to use a 2MiB I/O space as specified
in the "72. Serial-ATA" section of R-Car-Gen3-rev0.52E.pdf

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>