ARM: shmobile: r8a7740: fixup DT machine desc name typo

r8a7740 machine desc name should be R8A7740,
not SH7372

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms@verge.net.au>