soc: ti_simplelink: kconfig: Refactor CCFG flash block handling

Instead of having TI_CCFG_PRESENT as a symbol that's only defined in
soc/arm/ti_simplelink/cc13x2_cc26x2/Kconfig.defconfig.series and y when
SOC_SERIES_CC13X2_CC26X2 is enabled, turn it into a helper symbol that's
selected by SOC_SERIES_CC13X2_CC26X2.

This avoids having a symbol that's only defined in a Kconfig.defconfig
file, which is confusing. It also makes things a bit more generic, in
case other boards with CCFGs are added.

Also rename it to HAS_TI_CCFG to be consistent with other helper
symbols, and add a help text.

Flagged by scripts/kconfig/lint.py.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>