at91: reorganize at91 subtargets

reorganizing at91 subtargets based on sama5 soc features and this fix
below problems.
  1. able to set neon flags to sama5d2 & sama5d4 subtargets.
  2. fix the make clean which removes all the subtargets in bin folder.
  3. able to configure kernel specific to subtarget.
  4. able to set vfpu4 flags to samad3 subtargets.

Signed-off-by: Sandeep Sheriker Mallikarjun <sandeepsheriker.mallikarjun@microchip.com>

target/linux/at91/sama5/config-default

@@ -1,52 +0,0 @@
-# CONFIG_ARCH_MULTI_CPU_AUTO is not set
-CONFIG_ARCH_MULTI_V6_V7=y
-CONFIG_ARCH_MULTI_V7=y
-CONFIG_ARM_HEAVY_MB=y
-CONFIG_ARM_L1_CACHE_SHIFT=6
-CONFIG_ARM_L1_CACHE_SHIFT_6=y
-# CONFIG_ARM_LPAE is not set
-CONFIG_ARM_PATCH_IDIV=y
-# CONFIG_ARM_THUMBEE is not set
-CONFIG_ARM_VIRT_EXT=y
-CONFIG_ATMEL_AIC5_IRQ=y
-CONFIG_CACHE_L2X0=y
-CONFIG_CPU_32v6K=y
-CONFIG_CPU_32v7=y
-CONFIG_CPU_ABRT_EV7=y
-# CONFIG_CPU_BPREDICT_DISABLE is not set
-CONFIG_CPU_CACHE_V7=y
-CONFIG_CPU_CACHE_VIPT=y
-CONFIG_CPU_COPY_V6=y
-CONFIG_CPU_HAS_ASID=y
-CONFIG_CPU_PABRT_V7=y
-CONFIG_CPU_TLB_V7=y
-CONFIG_CPU_V7=y
-CONFIG_DCACHE_WORD_ACCESS=y
-CONFIG_GENERIC_PINCONF=y
-CONFIG_HAVE_ARCH_BITREVERSE=y
-CONFIG_HAVE_ARM_SMCCC=y
-CONFIG_HAVE_AT91_GENERATED_CLK=y
-CONFIG_HAVE_AT91_H32MX=y
-CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y
-CONFIG_HAVE_SMP=y
-CONFIG_MIGHT_HAVE_CACHE_L2X0=y
-CONFIG_MMC_SDHCI=y
-CONFIG_MMC_SDHCI_IO_ACCESSORS=y
-CONFIG_MMC_SDHCI_OF_AT91=y
-CONFIG_MMC_SDHCI_PLTFM=y
-CONFIG_OUTER_CACHE=y
-CONFIG_OUTER_CACHE_SYNC=y
-CONFIG_PINCTRL_AT91PIO4=y
-CONFIG_PL310_ERRATA_588369=y
-CONFIG_PL310_ERRATA_727915=y
-CONFIG_PL310_ERRATA_753970=y
-CONFIG_PL310_ERRATA_769419=y
-CONFIG_POWER_RESET_AT91_SAMA5D2_SHDWC=y
-CONFIG_SOC_SAMA5=y
-CONFIG_SOC_SAMA5D2=y
-CONFIG_SOC_SAMA5D3=y
-CONFIG_SOC_SAMA5D4=y
-CONFIG_SOC_SAM_V7=y
-CONFIG_SPLIT_PTLOCK_CPUS=4
-# CONFIG_SWP_EMULATE is not set
-# CONFIG_THUMB2_KERNEL is not set