MAINTAINERS: Fix ALTERA SOCFPGA Files

Replace arch/arm/cpu/armv7/socfpga/ path with
arch/arm/mach-socfpga/ and removed board file path
since board/altera has different boards with relevant
board maintainers.

Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Jagan Teki <jagan@openedev.com>