[arm] Rename arm_cache_is_sp_register to arm_is_alternative_sp_register

All usages of this helper are really made to check if the register is
one of the alternative SP registers (MSP/MSP_S/MSP_NS/PSP/PSP_S/PSP_NS)
with the ARM_SP_REGNUM case being handled separately.

Signed-off-by: Luis Machado <luis.machado@arm.com>
Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Signed-off-by: Yvan Roux <yvan.roux@foss.st.com>