ARM: shmobile: r8a7778: correct model name in Kconfig

The correct model name is R-Car M1A or R8A77781; R8A77780 corresponds to R-Car
M1S which is a SH based SoC.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
[horms+renesas@verge.net.au: manually applied]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>