Kconfig: Unify endian support option

Move SUPPORT_BIG_ENDIAN, SUPPORT_LITTLE_ENDIAN to top-level
arch Kconfig and let architectures select them as necessary.

Remove if guard for Endianness selection choice so we can
have one of SYS_BIG_ENDIAN, SYS_LITTLE_ENDIAN config symbol
defined even on single endian system.

Default endian to SYS_BIG_ENDIAN for MIPS || MICROBLAZE
and LITTLE_ENDIAN for the rest to retain old config
behaviour.

Note: PPC, SH, Xtensa are technically bi-endian, but I
checked compiled u-boot image with readelf, U-Boot currently
only support little endian for SH and Xtensa, Big Endian for
PPC.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>