hexagon: select FRAME_POINTER instead of redefining it

Because FRAME_POINTER is defined in lib/Kconfig.debug, the arch Kconfig
should select it.

Add 'select FRAME_POINTER' to HEXAGON. ARCH_WANT_FRAME_POINTERS must
also be selected to avoid the unmet dependency warning.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Brian Cain <bcain@quicinc.com>